Optical camouflage filters

ABSTRACT

An article includes an optical filter that comprises a wavelength selective reflective layer and at least one wavelength selective absorbing layer. The optical filter has visible transmittance between 400 nm-700 nm of less than about 30% and near infrared transmittance at 830 nm-900 nm greater than about 30%.

RELATED APPLICATIONS

This application is related to U.S. Ser. No. 62/281,643 filed on Jan.21, 2016 and PCT Application No. PCT/CN2016/081271 filed on May 6, 2016,which are hereby incorporated by reference in their entireties.

BACKGROUND

Light may reflect from surfaces in different ways, for example, as aspecular reflection or as a diffusive reflection. In opaque materials,specular reflection may occur on an uppermost surface layer of thematerial, for example, at an air/material interface, and the reflectionmay carry a full spectrum of incident light. Specular reflection maymanifest as shininess or gloss, which may account for less than 4% ofthe total reflected light. In contrast, diffusive reflection may occurunder a top surface of the material, and may carry selected wavelengthsor color. For example, color may be seen in the diffuse reflection of anon-metallic object. Both kinds of reflection may be observed, forexample, at hybrid surfaces such as surfaces including a paint coatcovered by a clear top coat. Thus, specular reflection may occur at theair/top coat interface, while diffuse reflection may occur at the topcoat/paint coat interface.

Optical filters are employed in a wide variety of applications such asoptical communication systems, sensors, imaging, scientific andindustrial optical equipment, and display systems. Optical filters mayinclude optical layers that manage the transmission of incidentelectromagnetic radiation, including light. Optical filters may reflector absorb a portion of incident light, and transmit another portion ofincident light. Optical layers within an optical filter may differ inwavelength selectivity, optical transmittance, optical clarity, opticalhaze, and index of refraction.

SUMMARY

In some embodiments, an article includes an optical filter thatcomprises a wavelength selective reflective layer and at least onewavelength selective absorbing layer. The optical filter has visibletransmittance between 400 nm-700 nm of less than about 30% and nearinfrared transmittance at 830 nm-900 nm greater than about 30%.

Some embodiments are directed to a printed article that includes anoptical filter. The optical filter includes a wavelength selectivereflective layer and at least one printed wavelength selective absorbinglayer. The optical filter has visible transmittance between 400 nm-700nm of less than about 30% and near infrared transmittance at 830 nm-900nm greater than about 30%.

According to some embodiments, a system includes one or both of a lightemitter and a light receiver and an optical filter adjacent one or bothof the light emitter and the light receiver. The optical filter includesa wavelength selective reflective layer and at least one wavelengthselective absorbing layer. The optical filter has visible transmittancebetween 400 nm-700 nm of less than about 30% and near infraredtransmittance at 830 nm-900 nm greater than about 30%.

In some embodiments, an article includes an optical filter. The opticalfilter comprises a wavelength selective reflective layer and at leastone wavelength selective absorbing layer having visible absorption at400 nm-700 nm greater than about 30%. The optical filter has nearinfrared transmittance at 830 nm-900 nm greater than about 30%.

According to some embodiments a system includes one or both of a lightemitter and a light receiver and an optical filter adjacent one or bothof the light emitter and the light receiver. The optical filter includesa wavelength selective reflective layer having near infraredtransmittance at 830 nm-900 nm greater than about 30%. The opticalfilter includes at least one wavelength selective absorbing layer havingvisible absorption at 400 nm-700 nm greater than about 30% and a nearinfrared transmittance at 830 nm-900 nm greater than about 30%.

In some embodiments an article includes an optical filter. The opticalfilter includes a wavelength selective scattering layer comprising atleast one of a dye and a pigment. The wavelength selective scatteringlayer scatters visible wavelengths between 400 nm-700 nm and transmitsnear-infrared wavelengths between 830 nm-900 nm. The optical filterfurther includes a wavelength selective reflective layer configured totransmit near-infrared wavelengths between 830 nm-900 nm.

Some embodiments are directed to a method of making an optical filterthat includes forming a wavelength selective absorbing layer and awavelength selective reflective layer. The wavelength selectiveabsorbing layer and a wavelength selective reflective layer are formedsuch that the optical filter has average visible transmittance forwavelengths between 400 nm-700 nm of less than about 30% and averagenear infrared transmittance for wavelengths between 830 nm to 900 nmgreater than about 30%.

The details of one or more aspects of the various embodiments are setforth in the accompanying drawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1K are lateral cross-sectional views of example articlesincluding optical filters.

FIGS. 2A-2F are conceptual and schematic diagrams of example systemsincluding an optical filter.

FIGS. 2G and 2H are front and back views, respectively, illustrating anarticle that includes an optical filter, wherein the article isconfigured to fit an object.

FIGS. 3A-3D are conceptual diagrams of an example system including anexample optical filter and an electronic display displaying a visiblyperceptible pattern and an invisible near-infrared pattern.

FIG. 4 is a flowchart of an example technique.

FIG. 5 is a photograph of an example article including an exampleoptical filter and an inked pattern.

FIG. 6A is a photograph of a solar panel. FIG. 6B is a photograph of asolar panel camouflaged by an example optical filter.

FIG. 7 is a photograph of an example article including an exampleoptical filter and an inked pattern.

FIGS. 8A-8C are photographs of an example system including an exampleoptical filter and a near-infrared LED.

FIG. 9 is an atomic force microscopy (AFM) photograph of a surface of anexample optical filter.

FIGS. 10A and 10B are scanning electron microscopy (SEM) photographs ofan example optical filter.

FIG. 11 is a chart presenting % reflectance and % transmittance versuswavelength for example optical filters.

FIGS. 12A and 12B are charts presenting % transmittance versuswavelength for example optical filters.

FIG. 13 is a chart presenting % transmittance versus wavelength forexample optical filters.

FIG. 14 is a chart presenting results of Mie scattering, showingscattering efficiency versus wavelength for example optical filters.

FIG. 15 is a chart presenting near-infrared scattering ratio as afunction of particle diameter and refractive index difference forexample wavelength selective scattering layers including a medium and aplurality of particles.

FIGS. 16A-16D are photographs comparing wet-out of a near-infrared filmthat includes a near-infrared black ink coating and a near-infrared filmthat does not include a near-infrared black ink coating.

FIG. 17 is a chart presenting % transmittance versus wavelength for thenear-infrared films of FIGS. 16A-16D.

FIGS. 18A-18B are photographs of example near-infrared films including acolored absorbing layer.

FIG. 19 is a chart presenting % transmittance versus wavelength for areflective multilayer optical film coated with a near-infraredantireflective coating compared to a reflective multilayer optical filmwithout a near-infrared antireflective coating.

FIG. 20A is a photograph of an example system including an infrared LEDwith a visible light component. FIG. 20B is a photograph of an examplesystem including an infrared LED with a visible light component filteredby a reflective multilayer optical film without an absorbing layer.

FIG. 21 is a chart presenting % transmittance versus wavelength for areflective multilayer optical film without an infrared dye coating.

FIG. 22 is a chart presenting % transmittance versus wavelength for areflective multilayer optical film with an infrared dye coating comparedto comparative optical filters without dye coatings.

FIG. 23 shows graphs of the transmittance at normal incidence for fivefilm samples of increasing dye loadings.

FIGS. 24 and 25 show the transmittance of dyes laminated to a mirrorfilm at normal incidence, 20, and 60 degrees from normal incidence.

FIG. 26 shows modeled graphs of samples at normal incidence with dyeconcentrations that vary from 3 to 34.7% laminated to an optimizedmirror film.

FIG. 27 shows the transmittance of light through a mirror at normalincidence.

FIG. 28 shows a plot of the same data as in FIG. 26 but with a scale of0 to 0.1% transmittance.

FIG. 29 shows the transmittance of a wavelength selective reflectivepolarizer.

FIG. 30 shows the transmittance of samples at normal incidence with dyeconcentrations that vary from 3 to 34.7% combined with the reflectivepolarizer of FIG. 29.

It should be understood that features of certain Figures of thisdisclosure may not necessarily be drawn to scale, and that the Figurespresent non-exclusive examples of the techniques disclosed herein.

DETAILED DESCRIPTION

In this disclosure, “visible” refers to wavelengths in a range betweenabout 400 nm and about 700 nm, and “near-infrared” refers to wavelengthsin a range between about 700 nm and about 2000 nm, for example,wavelengths in a range between about 800 nm and about 1200 nm. ULI(ultra-low index) films refers to optical films including a binder, aplurality of particles, and a plurality of interconnected voids, asdescribed in U.S. Patent Application Publication No. 2012/0038990,incorporated herein in its entirety by reference.

Ambient sources of electromagnetic radiation may interfere withreceivers configured to receive light of particular wavelengths or fromparticular sources, or with light emitters configured to emit light ofparticular wavelengths. For example, visible wavelengths may interferewith receiving, sensing, or transmitting near-infrared wavelengths, forexample, by increasing noise in a light receiver or in a light emitter.Sources of electromagnetic radiation may also be unintentionallyrevealed. For example, while light emitted by a light emitter configuredto emit only near-infrared wavelengths may not be visibly perceptible,the device or the structure responsible for emitting the light, forexample, a housing of the light emitter, may be visibly perceptible.Masking, concealing or otherwise camouflaging the light emitter maypresent challenges because the camouflage techniques may undesirablyresult in blocking, interference, or reduction in the transmission ofdesired near-infrared wavelengths. Optical filters according to examplesof this disclosure may be used to prevent unwanted optical interferencefrom visible wavelengths, or to camouflage sources of electromagneticradiation from visible perception, while at least partially allowingdesired near-infrared wavelengths to be transmitted by a light emitteror received by a light receiver, or while allowing transmission ofnear-infrared wavelengths with relatively high clarity.

For example, a light receiver operating to receive or sensenear-infrared wavelengths may be shielded from visible wavelengths,preventing interference with the receiving or sensing of near-infraredwavelengths that may be caused by visible wavelengths. A lighttransmitter operating to transmit near-infrared wavelengths may becamouflaged against visible perception by scattering visiblewavelengths. For example, the scattered visible wavelengths may concealthe presence of the light transmitter, without obstructing thetransmission of near-infrared wavelengths.

The amount of specular reflection off a surface may be determined byFresnel reflection of air interface. For an opaque surface with a cleartop layer, it may be assumed that all specular reflection arises fromthe top air interface, and that the rest of the reflection is diffusivereflection from a bottom layer. An opaque colored material could alsofollow similar model, while using its refractive index to calculateFresnel reflection on top surface and treat all other reflection isdiffusive. The example optical filters may have a diffusive coatingdisposed on a clear substrate or a reflective film. When the diffusivecoating is coated on clear substrate, it may have a higher haze to hidethe items underneath. When the coating is coated on a reflector, thecoating will diffuse incident light twice, by reflection. In that case,the coating may have less haze.

Thus example systems may include one or both of a light receiver and alight emitter, and an optical filter that includes a wavelengthselective scattering layer that may at least partially reduce thetransmission of visible wavelengths, while at least partially allowingthe transmission of near-infrared wavelengths. For example, thewavelength selective scattering layer may scatter a majority of incidentvisible light. Example systems and articles according to the presentdisclosure may include example optical articles including examplewavelength selective scattering layers that transmit near-infrared lightwith relatively high clarity while reducing the transmission of visiblewavelengths, for example, by selectively scattering or reflectingvisible wavelengths.

FIGS. 1A-1K are lateral cross-sectional views of example articlesincluding optical filters. FIG. 1A shows a lateral cross-sectional viewof example article 10 a. Article 10 a includes a substrate 12 and awavelength selective scattering layer 14. The substrate 12 may includeglass, polymer, metal, or any other suitable rigid, semi-rigid, or softmaterials, and combinations thereof. While the substrate 12 is shown asa layer in the example article 10 a of FIG. 1A, in examples, substrate12 may assume any suitable three dimensional shape that may have a flat,a substantially flat, or a textured surface. In examples, substrate 12may include a housing, a screen, a part, or a surface of a device, forexample, of an electronic device such as a personal computing orcommunication device, for example, a cellphone or a smartwatch. In someembodiments, the substrate 12 may be flexible. In some embodiments, thesubstrate 12 may comprise glass or a polymer in some embodiments.

One or more layers of the optical filter may be laminated or adhesivelyattached to the substrate 12 or may be integrally formed on thesubstrate 12. In some embodiments, the substrate 12 may be a moldedcomponent. In some embodiments, the substrate 12 may be a molded part.One or more layers of the optical filter, e.g., one or more of thewavelength selective layers 14, 16, 34 may be attached to the substrate12 during an insert injection molding process. For example, thewavelength selective layers 14, 16, 34 (and/or other layers of theoptical filter) may be placed into an injection mold prior to molding.Subsequent to placing the layers into the mold, the mold material isinjected into the injection mold to form the molded substrate. Theinjection molded substrate with the optical filter layers attachedthereto is then removed from the mold.

The optical filter according to any of the examples 10 a-10 k shown inFIGS. 1A through 1K may be formed in a two dimensional or threedimensional shape. In some embodiments, one or more of the wavelengthselective layers 14, 16, 34 (and/or other layers of the optical filter)may be formed in a three dimensional shape before or after beingdisposed on and/or attached to the substrate 12. The optical filteraccording to any of the examples 10 a-10 k shown in FIGS. 1A through 1Kmay be flexible. The optical filter according to any of the examples 10a-10 k shown in FIGS. 1A through 1K may include various features,including slots, holes, protrusions, and/or other features.

The wavelength selective scattering layer 14 selective scatters visiblelight and transmits near-infrared light. In examples, the wavelengthselective scattering layer may have a near-infrared scattering ratio ofless than about 0.9, less than about 0.8, less than about 0.7, less thanabout 0.6, or less than about 0.5. The near-infrared scattering ratio isa ratio of an average near-infrared scattering to an average visiblescattering. For example, the average scattering in a selected narrow orbroad near-infrared wavelength band (for example, of bandwidth 1300 nm,500 nm, 100 nm, 10 nm, 1 nm) may be determined, and the averagescattering in a selected narrow or broad visible wavelength band may bedetermined, and a ratio of the respective averages may be determined. Inexamples, the wavelength selective scattering layer 14 may have avisible reflective haze ratio of greater than about 0.5, or greater thanabout 0.7, or greater than about 0.9. The visible reflective haze ratiois a ratio of an average visible diffusive reflectance to an averagevisible total reflectance. In examples, the wavelength selectivescattering layer 14 may transmit less than about 50% of incident visiblelight. In examples, the wavelength selective scattering layer 14 maytransmit greater than about 50% of incident near-infrared light. Inexamples, the wavelength selective scattering layer 14 may transmit lessthan about 50% of incident visible light, and transmit greater thanabout 50% of incident near-infrared light. In examples, the wavelengthselective scattering layer 14 may scatter greater than about 50% ofincident visible light. For example, the wavelength selective scatteringlayer 14 may transmit less than about 50% of incident visible light byscattering more than about 50% of incident visible light. In examples,the wavelength selective layer 14 may scatter greater than about 50% ofincident visible light as white light.

The wavelength selective scattering layer 14 may include a medium and aplurality of particles with respective predetermined refractive indices.In examples, the wavelength selective scattering layer 14 may include abeaded diffuser layer. For example, the wavelength selective scatteringlayer 14 may include a medium and beads dispersed in the medium. Themedium of the beaded diffuser layer may include glass, polymer, or anyother suitable optical medium, or combinations thereof. The beads mayinclude silica, glass, polymeric, organic, inorganic, metal oxide,polystyrene, or other suitable scattering materials, or combinationsthereof. The diffuser layer may include pores including a gas such asair. In examples, the pores including gas may be encapsulated in beads.

The wavelength selective scattering layer 14 may include an opticalmedium have a first refractive index. The optical medium may include aplurality of particles. The plurality of particles may have a secondrefractive index such that an absolute difference between the firstrefractive index and the second refractive index is less than about 0.1.In examples, the plurality of particles may have an average particlesize of less than about 5 μm, and the absolute difference between thefirst and second refractive indices may be less than about 0.1. Inexamples, the plurality of particles may have an average particle sizeof less than about 1 μm, and the absolute difference between the firstand second refractive indices may be less than about 0.2. In examples,the plurality of particles may have an average particle size of lessthan about 0.5 μm, and the absolute difference between the first andsecond refractive indices may be less than about 0.4. In examples, theplurality of particles may have an average particle size of less thanabout 0.3 μm, and the absolute difference between the first and secondrefractive indices may be less than about 0.6. In examples, theplurality of particles may have an average particle size of less thanabout 0.2 μm, and the absolute difference between the first and secondrefractive indices may be less than about 1.8.

In examples, an average particle size of the plurality of particles, thefirst refractive index, and the second refractive index are selectedfrom a region under line 82 of FIG. 15, described below. Thus, thenear-infrared scattering ratio of the wavelength selective scatteringlayer 14 may be less than 0.2. In examples, an average particle size ofthe plurality of particles, the first refractive index, and the secondrefractive index are selected from a region under line 84 of FIG. 15.Thus, the near-infrared scattering ratio of the wavelength selectivescattering layer 14 may be less than 0.4. In examples, an averageparticle size of the plurality of particles, the first refractive index,and the second refractive index are selected from a region under line 86of FIG. 15. Thus, the near-infrared scattering ratio of the wavelengthselective scattering layer 14 may be less than 0.6. In examples, anaverage particle size of the plurality of particles, the firstrefractive index, and the second refractive index are selected from aregion under line 88 of FIG. 15. Thus, the near-infrared scatteringratio of the wavelength selective scattering layer 14 may be less than0.8. In examples, the near-infrared scattering ratio of the wavelengthselective scattering layer 14 may be less than 0.7, or may be less than0.5. In examples, the region under respective lines 82, 84, 86, 88 orany other region may be bounded by a lower particle size bound. Forexample, the region may only include particle sizes above 10 nm, or 30nm, or 50 nm, or particle sizes greater than particle sizes at whichRayleigh scattering may manifest or predominate.

In examples, the wavelength selective scattering layer 14 may have atotal visible reflectance of less than 50%, of at least 50%, or at least60%, or at least 70%. In examples, the total visible reflectance may beless than 50%, and the wavelength selective scattering layer 14 mayconceal objects by visible haze. In examples, the total visiblereflectance may be greater than 50%, and the wavelength selectivescattering layer 14 may conceal objects by a combination of visiblereflection and visible haze. In examples, the wavelength selectivescattering layer 14 may have an average near-infrared scattering of lessthan 60%, or less than 40%. In examples, the wavelength selectivescattering layer may have an average visible scattering of greater than10%, or greater than 25%, or greater than 58%. In examples, a differencebetween the % total visible reflectance and the % diffuse visiblereflectance of the wavelength selective scattering layer 14 may be lessthan 20. In examples, the wavelength selective scattering layer may havean average near-infrared scattering of less than 40%, and an averagevisible scattering of greater than 58%, and the difference between the %total visible reflectance and the % diffuse visible reflectance may beless than 18.

In examples, the wavelength selective scattering layer 14 may have avisible haze of at least 15%, or at least 25%, or at least 35%, or atleast 50%. In examples, the optical filter 10 a may include surfaceoptical microstructures, such as microreplicated surface structures.

In examples, the wavelength selective scattering layer 14 may includeULI layer including a binder, a plurality of particles, and a pluralityof interconnected voids. A volume fraction of the plurality ofinterconnected voids in the optical filter may not less than about 20%.A weight ratio of the binder to the plurality of the particles may notbe less than about 1:2.

The wavelength selective scattering layer 14 may be configured totransmit near-infrared wavelengths, e.g., wavelengths between 830 nm-900nm, between 900 nm and 980 nm, and/or between 800 nm and 1200 nm and toscatter at least visible wavelengths, e.g., wavelengths between 400 nmto 700 nm. The wavelength selective scattering layer 14 may include oneor both of a dye and a pigment that scatters light. For example,wavelength selective scattering layer 14 may comprise a coating thatincludes the dye and/or pigment. The dye and/or pigment may contain morethan about 11%, more than about 12%, more than about 13%, or even morethan about 14% solids. The dye and/or pigment of the wavelengthselective scattering layer 14 may comprise one or both of a black dyeand/or pigment and a color dye and/or pigment, e.g., a cyan, magenta,and/or yellow color dye or pigment. In some embodiments, the absorber,e.g., dye or pigment, can be one absorber material or can be acombination of more than one absorber material. For example, multipledyes, pigments and/or other absorber materials can be combined in anyway, e.g., mixed together and/or layered one on top of another, etc.

FIG. 1B shows a lateral cross-sectional view of example article 10 b.Article 10 b may include the substrate 12, the wavelength selectivescattering layer 14, and a reflective layer 16. While reflective layer16 is shown between the wavelength selective scattering layer 14 and thesubstrate 12 in article 10 b, in examples, article 10 b may not includethe substrate 12, and the wavelength selective scattering layer may bedisposed on the reflective layer 16. In examples, substrate 12 mayinclude the reflective layer 16, for example, at a major surface orwithin an interior of substrate 12. In examples, the reflective layer 16may be disposed below the substrate 12. In examples, the reflectivelayer 16 may be disposed above the substrate 12. In examples, thereflective layer 16 may be perforated. In examples, article 10 b mayreflect less than 50% of visible light, and transmit more than 50% ofnear-infrared light. In examples, reflective layer 16 may be wavelengthselective, for example, reflecting only selected wavelengths. Reflectivelayer 16 may include a multilayer optical film, a dichroic reflector, aninterference film, an inorganic multilayer stack, a metal dielectricstack, a polished substrate, a mirror, a reflective polarizer, or areflective surface such as a reflective metal or glass surface. Inexamples, article 10 b may include a dye layer (not shown) between thereflective layer and the wavelength selective scattering layer 14, orabove the wavelength selective scattering layer 14, or positionedadjacent any layer in article 10 b. The dye layer may include aspectrally selective dye that may be transmissive or clear innear-infrared, and neutral in visible, such that it reduces the visiblereflection of the reflective layer 16. In examples, the dye layer mayhave at least 30%, 50%, 70%, or 90% absorption. In examples, the dyelayer could be colored, so that it has a visible color, while remainingtransmissive in near-infrared.

FIG. 1C shows a lateral cross-sectional view of example article 10 c.Article 10 c may include the substrate 12 and the wavelength selectivescattering layer 14. Article 10 c may optionally include one or more ofthe reflective layer 16, an ink receptive layer 18, a printed patternlayer 22, and a protective layer 24, as shown in FIG. 1C. While FIG. 1Cshows a particular arrangement for layers in article 10 c, therespective layers may be rearranged in any suitable configuration. Forexample, substrate 12 may be omitted when the reflective layer 16 ispresent. The protective layer 24 may include a sealant layer. Inexamples, the inked pattern layer 22 includes a printed pattern of inkor pigment that may be deposited on the ink receptive layer 18. Inexamples, the ink receptive layer may be omitted, and the inked patternlayer 22 may be deposited on the wavelength selective scattering layer14. In examples, the protective layer 24 may be disposed between theinked pattern layer 22 and the wavelength selective scattering layer 14.In examples, two protective layers 24 may be disposed, one above theinked pattern layer 22, and another adjacent the wavelength selectivescattering layer 14.

FIG. 1D shows a lateral cross-sectional view of example article 10 d.Article 10 d may include the substrate 12, the wavelength selectivescattering layer 14, a first sealant layer 26 and a second sealant layer28. One of both of the first sealant layer 26 and the second sealantlayer 28 may include a latex coating. The respective sealant layers mayprotect the integrity of the wavelength selective scattering layer 14,for example, by preventing or reducing the intrusion of moisture orother reactants or disintegrants. The respective sealant layers may alsoprovide structural support and physical stability to the wavelengthselective scattering layer 14. For example, one or both of the firstsealant layer 26 and the second sealant 28 may allow the wavelengthselective scattering layer 14 to be peeled off or removed from amanufacturing substrate and then transported to and applied over aproduct substrate, for example, over substrate 12.

FIG. 1E shows a lateral cross-sectional view of example article 10 e.Article 10 e may include the substrate 12, the wavelength selectivescattering layer 14 adjacent the substrate 12, and an inked patternlayer 24 deposited on the wavelength selective scattering layer 14. Asensor layer 32 including respective sensor segments 32 a, 32 b, 32 c,and 32 d may be disposed adjacent the substrate 12. In examples, thesubstrate 12 may be omitted, and the wavelength selective scatteringlayer 14 may be deposited on the sensor layer 32. In examples, thewavelength selective scattering layer 14 may include respectiveselective scattering segments 14 a, 14 b, 14 c, and 14 d that may bealigned with respective sensor segments 32 a, 32 b, 32 c, and 32 d. Oneor more of the selective scattering segments may be omitted, so that thewavelength selective scattering layer 14 may include at least oneperforation that may be aligned with at least one of the respectivesensor segments. Thus different selective scattering segments may betuned by changing the near-infrared scattering ratio, the visible hazeratio, or other optical properties that may improve the performance ofthe sensor segment aligned with the respective selective scatteringsegment. While four segments are shown in the wavelength scatteringlayer 14 and the sensor layer 32 of FIG. 1E, in examples, the wavelengthscattering layer 14 and the sensor layer 32 may have any suitable numberof segments. While sensor layer 32 is described in the example of FIG.1E, in examples, article 10 e may include light sources 32 a, 32 b, 32c, and 32 d instead of sensor segments.

FIG. 1F shows a lateral cross-sectional view of example article 10 f.Article 10 f may include the substrate 12, the wavelength selectivescattering layer 14, the reflective layer 16, and a wavelength selectiveabsorbing layer 34. The reflective layer 16 may include a wavelengthselective reflective layer. For example, the reflective layer 16 mayinclude a wavelength selective interference filter or a wavelengthselective multilayer optical film. In some examples, the wavelengthselective absorbing layer 34 may include any suitable dye or pigmentthat has a greater infrared transmittance than a visible transmittance,for example, a near-infrared black ink that substantially absorbsvisible wavelengths while transmitting near-infrared wavelengths. Forexample, the wavelength selective absorbing layer 34 may include dyes orinks such as Spectre™ inks, for example Spectre™ 100, 110, 120, 130,140, 150, or 160 (Epolin, Newark, N.J.); Mimaki inks, for example MimakiES3, SS21, BS3, SS2, or HS (Mimaki Global, Tomi-city, Nagano, Japan); orSeiko inks, for example Seiko 1000, 1300, SG700, SG740, or VIC (SeikoAdvance Ltd., Japan). In examples, the wavelength selective absorbinglayer 34 may include one or more of cyan, magenta, yellow, or black dyecomponents, or may include a dye having any desired color, for example,by scattering or reflecting a predetermined wavelength band, peak, orspectrum associated with a predetermined color. In some examples, thewavelength selective absorbing layer 34 may include a spectrallyselective multilayer absorbing film that may have a greater infraredtransmittance than a visible transmittance. In examples, a color of thewavelength selective absorbing layer 34 may be selected to tune theappearance of the article 10 f as a whole, for example, to tune thereflected or scattered wavelengths so as to modify the apparent color ofthe article 10 f exhibited by a major surface of the article 10 f. Thewavelength selective absorbing layer 34, while blocking visiblewavelengths, may transmit at least some, or substantially all,near-infrared wavelengths. In some examples, the wavelength selectiveabsorbing layer 34 may include a separate coating including one or bothof a dye or a pigment. In some examples, the wavelength selectiveabsorbing layer 34 may not include a dye, and may include anear-infrared transmissive visible blocking pigment. For example, thewavelength selective absorbing layer 34 may include Lumogen® Black FK4280 or Lumogen Black FK 4281 (BASF, Southfield, Mich.). In someexamples, the wavelength selective absorbing layer 34 may include amultilayer film, one or more of the layers of the multilayer filmincluding one or both of a dye or a pigment. In some examples, thewavelength selective absorbing layer 34 may include or be an adhesivelayer, a polymer layer, a skin layer, or any other layer of a multilayerfilm that includes a dye or a pigment. In some examples, article if maynot include a separate wavelength selective absorbing layer 34, andinstead may include a wavelength selective dye or a pigment in any othersuitable layer. In some examples, the wavelength selective absorbinglayer 34 or any other layer of article 10 f may only include dye orpigment in a predetermined pattern or region. In some examples. Thewavelength selective absorbing layer 34 may exhibit broadbandabsorption, for example, absorption over a predetermined wavelengthband, by including one or more absorbing dyes or pigments that absorb atleast a respective sub-band of the predetermined wavelength band.

In some examples, the wavelength selective absorbing layer 34 mayinclude a beads or particles to be exhibit diffusing or scattering. Forexample, the wavelength selective absorbing layer 34 may include amedium and beads or particles dispersed in the medium. The medium mayinclude glass, polymer, or any other suitable optical medium, orcombinations thereof. The beads or particles may include silica, glass,polymeric, organic, inorganic, metal oxide, polystyrene, or othersuitable scattering materials, or combinations thereof. The wavelengthselective absorbing layer 34 may include diffusive or scattering voidsor pores, and the voids or pores may include a gas such as air.

Thus, each respective wavelength selective layer (14, 16, 34) maytransmit near-infrared wavelengths. For example, one or more of thewavelength selective layers, or the article 10 f as a whole may have anear-infrared transmittance, for example, transmittance at wavelengthsgreater than 830 nm, of greater than 5%, or greater than 10%, or greaterthan 20%, of greater than 50%, or greater than 7%. In examples, article10 f may transmit less than 5%, or less than 1%, or about 0. Inexamples, article 10 f may have a near-infrared transmittance of greaterthan 10% for wavelengths greater than 830 nm. In examples, article 10 fmay have a near-infrared transmittance of greater than 20% forwavelengths greater than 850 nm. In examples, article 10 f may have anear-infrared transmittance of greater than 50% for wavelengths greaterthan 870 nm. In examples, article 10 f may have a near-infraredtransmittance of greater than 50% for wavelengths greater than 900 nm.In examples, article 10 f may have an average near-infraredtransmittance of greater than 75% for wavelengths greater than 900 nm.

In some examples, as shown in FIG. 1F, the wavelength selectiveabsorbing layer 34 may be between the wavelength selective scatteringlayer 14 and the wavelength selective reflective layer 16. Positioningthe wavelength selective absorbing layer 34 behind the wavelengthselective scattering layer 14 may be used to tune the grey scale orapparent whiteness of the wavelength selective scattering layer 14. Asdiscussed above, the wavelength selective absorbing layer 34 may includea non-neutral color to tune visual appearance, for example, a colorcoordinate in a predetermined color space. In examples, the wavelengthselective absorbing layer 34 may reduce a total visible reflectance ofthe optical filter by a predetermined magnitude without substantiallyreducing a total near-infrared transmittance. While example article 10 fincludes a separate wavelength selective absorbing layer 34, in someexamples, for example, example article 10 g of FIG. 1G, a wavelengthselective dye may be added to the wavelength selective scattering layer14 g so that wavelength selective scattering layer also acts as anabsorbing layer. In examples, the wavelength selective scattering layer14 may be disposed on top of dyed wavelength selective scattering layer14 g.

In some examples, as shown in FIG. 1H, an example article 10 h mayinclude the wavelength selective reflective layer 16 positioned betweenwavelength selective scattering layer 14 and wavelength selectiveabsorbing layer 34. The wavelength selective absorbing layer 34 mayreduce a total visible reflectance uniformly over an area of a majorsurface of the article 10 h, without substantially reducing a totalnear-infrared transmittance. The uniform reduction in total visiblereflectance may be used to reduce or prevent wet-out. Wet-out is aphenomenon that may arise from visible light leakage or transmission ofvisible light through all layers of article 10 h, which may result inthe appearance of visible discontinuities, disruptions, aberrations,variations, or disturbance in the uniform appearance of an opticalfilter. For example, regions at which an optical filter contacts anunderlying substrate, may exhibit wet-out, whereby a shape correspondingto the region of contact may be perceptible through the optical filter.The wavelength selective absorbing layer 34 may uniformly reduce visiblereflectance over an entire area of the article 10 h, and prevent visiblelight leakage, while still allowing near-infrared wavelengths to betransmitted, such that no discontinuities or disturbances are visibleacross a major surface of the article 10 h, thus avoiding wet-out.

In some examples, the wavelength selective absorbing layer 34 may occupya complete intermediate area adjacent a major surface of wavelengthselective reflective layer 16. However, in some examples, as shown inFIG. 1H, the wavelength selective absorbing layer 34 may occupy apartial region adjacent a major surface of the wavelength selectivereflective layer 16, with a light diffusive layer 36 occupying theremaining regions adjacent the major surface of the wavelength selectivereflective layer 16. This configuration may be used to reduce the amountof near-infrared dye that may be required to create the wavelengthselective absorbing layer 34, for example, where a relatively dark orvisible light absorbing component may be placed adjacent the lightdiffusive layer 36. In examples where a visible light absorbingcomponent, for example, a sensor, is disposed adjacent a region of thewavelength selective reflective layer 16, no wet-out may be expected tomanifest in that region. Therefore, covering that region with thewavelength selective absorbing layer 34 may not be necessary, andinstead, the light diffusive layer 36 may be used adjacent thatcomponent, for example, reducing costs associated with near-infrareddye.

In some examples, example articles may not include the wavelengthselective scattering layer 14, and may only include the wavelengthselective reflective layer 16 and the wavelength selective absorbinglayer 34, as shown in FIGS. 1I to 1K. In some examples, as shown in FIG.1I, an example article 10 i may include the wavelength selectivereflective layer 16 disposed adjacent the substrate 12, with thewavelength selective absorbing layer 34 between the substrate 12 and thewavelength selective reflective layer 16. In various embodiments theorder of the layers in FIGS. 1A through 1K may change. In someembodiments, one or more intervening layers may be disposed between anyof the layers of articles 10 a through 10 k illustrated in FIGS. 1Athrough 1K. For example, an intervening layer may be disposed betweenthe wavelength selective scattering layer 14 and the wavelengthselective reflective layer 16, between the wavelength selectivescattering layer 14 and the wavelength selective absorbing layer 34and/or between the wavelength selective absorbing layer 34 and thewavelength selective reflective layer 16, etc.

In some examples, as shown in FIG. 1J, an example article 10 j mayinclude the wavelength selective reflective layer 16 disposed adjacentthe substrate 12, with the wavelength selective reflective layer 16between the wavelength selective absorbing layer 34 and the substrate12. In some examples, as shown in FIG. 1K, example article 10 k mayinclude the wavelength selective reflective layer 16 between a firstwavelength selective absorbing layer 34 a and a second wavelengthselective absorbing layer 34 b. The wavelength selective absorbinglayers 34, 34 a, and 34 b may be used to compensate for nonuniformblocking of visible wavelengths by the wavelength selective reflectivelayer 16. For example, while the wavelength selective reflective layer16 may block the transmission of a majority of visible wavelengths, thewavelength selective reflective layer 16 may still allow peaks or bandsof certain visible wavelengths to pass through. Thus, wavelengthselective reflective layer 16 may “leak” some visible light, which mayreveal objects to be concealed by the wavelength selective reflectivelayer 16, for example, from visual perception. A wavelength selectivedye can be selected to block at least those visible wavelengthstransmitted by the wavelength selective reflective layer 16, so that theexample articles 10 i-10 k substantially block all visible wavelengthswhile transmitting near-infrared wavelengths.

In examples, articles 10 i-10 k may have average visible transmittancefor wavelengths between 380-800 nm or for wavelengths between 400 nm-700nm of less than 0.1% and average near-infrared transmittance forwavelengths between 830 nm-900 nm, between 900 nm-980 nm, and/or between800 nm-1200 nm of greater than 50%. As denoted herein an average visibletransmittance of a wavelength range is the average value of thetransmittance of all wavelengths within the range. In examples, articles10 i-10 k may have average visible transmittance for wavelengths between380-800 nm or between 400 nm-700 nm of less than 0.01% and averagenear-infrared transmittance for wavelengths between 830 nm-900 nm, 900nm-980 nm, and/or between 800 nm-1200 nm of greater than 75%. Articles10 i-10 k may have visible transmittance for all wavelengths between380-800 nm or for all wavelengths between 400 nm-700 nm of less than0.1% and near-infrared transmittance for all wavelengths between 830nm-900 nm, between 900 nm-980 nm, and/or between 800 nm-1200 nm ofgreater than 50%. In examples, articles 10 i-10 k may have visibletransmittance for wavelengths between 380 nm-800 nm or between 400nm-700 nm of less than 0.01% and near-infrared transmittance forwavelengths between 830 nm-900 nm, 900 nm-980 nm, and/or between 800nm-1200 nm of greater than 75%. In examples, example articles 10 i-10 kmay further include a sealant layer or a protective layer, as discussedabove with reference to FIGS. 1A-1E.

In some embodiments, the articles 10 f-10 k including a wavelengthselective absorbing layer 34 may have average visible transmittance forwavelengths between 400 nm-700 nm of less than about 30% and averagenear infrared transmittance for wavelengths between 830 nm-900 nm,between 900 nm-980 nm, and/or between 800 nm-1200 nm greater than about30%. In some embodiments, average visible transmittance of the articles10 f-10 k for wavelengths between 400 nm and 700 nm may be less thanabout 20%, less than about 10%, less than about 5%, less than about 2%,or less than about 1%. In some embodiments, average near infraredtransmittance of articles 10 f-10 k for wavelengths between 830 nm-900nm, between 900 nm-980 nm, and/or between 800 nm-1200 nm may be greaterthan about 40%, greater than about 50%, or greater than about 75%. Insome embodiments, the articles 10 f-10 k including a wavelengthselective absorbing layer 34 may have visible transmittance for allwavelengths between 400 nm-700 nm of less than about 30% and nearinfrared transmittance for all wavelengths between 830 nm-900 nm,between 900 nm-980 nm, and/or between 800 nm-1200 nm greater than about30%. In some embodiments, visible transmittance of the articles 10 f-10k for all wavelengths between 400 nm and 700 nm may be less than about20%, less than about 10%, less than about 5%, less than about 2%, orless than about 1%. In some embodiments, near infrared transmittance ofarticles 10 f-10 k for all wavelengths between 830 nm-900 nm, between900 nm-980 nm, and/or between 800 nm-1200 nm may be greater than about40%, greater than about 50%, or greater than about 75%.

The wavelength selective absorbing layer 34 may have average visibleabsorption, e.g., for wavelengths between 400 nm-700 nm, greater thanabout 30%, greater than about 40%, greater than about 50%, greater thanabout 70%, or greater than about 90% in some embodiments. The wavelengthselective absorbing layer 34 may have visible absorption, e.g., for allwavelengths between 400 nm-700 nm, greater than about 30%, greater thanabout 40%, greater than about 50%, greater than about 70%, or greaterthan about 90% in some embodiments. The wavelength selective absorbinglayer 34 may have average near infrared transmittance for wavelengthsbetween 830 nm-900 nm, 900 nm-980 nm, and/or 800 nm-1200 nm greater thanabout 30%, greater than about 40%, or greater than about 50%. Thewavelength selective absorbing layer 34 may have near infraredtransmittance for all wavelengths between 830 nm-900 nm, 900 nm-980 nm,and/or 800 nm-1200 nm greater than about 30%, greater than about 40%, orgreater than about 50%.

The effects of the angle of incidence of light for articles including awavelength selective absorbing layer are discussed below, particularlywith reference to Example 22. Note that the angle of minimum visibletransmittance of light for an optical filter as discussed herein may ormay not be normal incidence. In some embodiments, the article may havevisible transmittance of light at normal incidence that is less thanvisible transmittance of light at an oblique angle. In some embodiments,the article has visible transmittance of light at an oblique angle,e.g., between 0 and 60 degrees, that is less than visible transmittanceof light at normal incidence.

The wavelength selective absorbing layer 34 can include one or both of awavelength selective dye and a wavelength selective pigment. In someimplementations, the wavelength selective absorbing layer may comprise aporous layer wherein the dye and/or pigment is disposed within pores ofthe porous layer.

The dye or pigment may absorb light in a first spectral range andre-emit light at a different second spectral range. For example, the dyeor pigment may comprise a fluorescing dye, phosphors, or quantum dotsthat absorb light at shorter wavelengths and re-emit light at longerwavelengths. As such, the dye or pigment may serve as a downconverter.For example, the dye or pigment may absorb ultraviolet wavelengths orblue wavelengths and may re-emit visible wavelengths. An optical filterthat includes a dye or pigment that absorbs and re-emits light atdifferent wavelengths can be disposed proximate to or attached to anobject to control the appearance of an object. For example, a spectralconverting layer, e.g., a downconverting layer, may boost the brightnessof white or may provide various special color effects for the object.

The optical filter may include more than one wavelength selectiveabsorbing layer as illustrated by article 10 k in FIG. 1K. For example,the article 10 k may include first and second wavelength selectiveabsorbing layers 34 a, 34 b wherein the first wavelength selectiveabsorbing layer 34 a has optical characteristics that are different fromthe optical characteristics of the second wavelength selective absorbinglayer 34 b. For example, the first wavelength selective absorbing layer34 a may include one or both of a black dye and a black pigment and thesecond wavelength selective absorbing layer 34 b may comprise one orboth of a color dye and a color pigment. In embodiments that include awavelength selective absorbing layer having a color dye or colorpigment, the dye or pigment may include one or more of cyan, magenta,and yellow components. Although FIG. 1K shows the wavelength selectivereflective layer 16 disposed between the first and second wavelengthselective absorbing layers 34 a, 34 b, this need not be the case. Insome embodiments, the layers may be arranged differently, e.g., thesecond wavelength selective absorbing layer may be disposed between thefirst wavelength selective absorbing layer and the wavelength selectivereflective layer. One or more intervening layers may be disposed betweenany of the layers of article 10 k illustrated in FIG. 1K, for example

As previously discussed, the optical filter represented by articles 10f-10 k in FIGS. 1F-1K may include a sealant layer and/or a protectivecoating in some embodiments. For example, the wavelength selectiveabsorbing layer may be disposed as a coating or printed patterned layeron another layer of the article 10 f-10 k. For example, the wavelengthselective absorbing layer may be coated or printed on the wavelengthselective reflective layer 16, the wavelength selective scattering layer14, the substrate 12, an ink receptive layer 18, a protective layer, 24,and/or a sealant layer 26, 28. In some embodiments, the wavelengthselective absorbing layer may be coated together with the wavelengthselective reflective layer 16, the wavelength selective scattering layer14, an ink receptive layer 18, a protective layer, 24, and/or a sealantlayer 26, 28. For example in some embodiments, a wavelength selectiveabsorbing material, e.g., dye and/or pigment, can be including in thewavelength selective scattering layer coating solution. In someembodiments, the wavelength selective absorbing material can be mixedinto the aqueous solution of a latex coating. The coating is thenapplied to the porous wavelength selective scattering layer. The aqueoussolution will drain into the pores of the scattering layer and dye thescattering layer with color. The latex particles will remain on thesurface of the combined scattering/absorbing layer and form a sealantlayer. In some embodiments, the dye or pigment may be combined in thesolvent of a wavelength selective scattering coating solution and thecombined solution coated into one layer that provides both scatteringand absorbing. coating can be applied When the wavelength selectiveabsorbing layer is a printed layer, it may be printed onto an underlyinglayer by screen printing, jet printing, flexographic printing and/orother types of printing, for example. In some embodiments, a printedarticle includes an optical filter comprising a wavelength selectivereflective layer and a printed wavelength selective absorbing layer. Amethod of making an optical filter having average visible transmittancefor wavelengths between 400 nm-700 nm of less than about 30% and averagenear infrared transmittance for wavelengths between 830 nm to 900 nmgreater than about 30%.comprising includes at least forming a wavelengthselective absorbing layer and a wavelength selective reflective layer.According to some aspects, forming the wavelength selective absorbinglayer and the wavelength selective reflective layer can include formingthe wavelength selective absorbing layer on the wavelength selectivereflective layer or forming the wavelength selective reflective layer onthe wavelength selective absorbing layer. In some embodiments, thewavelength selective absorbing layer and the wavelength selectivereflective layer may be formed as a single combined layer. Forming thewavelength selective absorbing layer and/or wavelength selectivereflective layer comprises printing or coating a wavelength selectiveabsorbing material and/or the wavelength selective reflective material.Printing or coating the wavelength selective absorbing layer and/or thewavelength selective reflective layer may include printing or coating asolution that includes two or more of a wavelength selective absorbingmaterial, a wavelength selective scattering material, a wavelengthselective reflective material, and a sealant material. In someembodiments, the wavelength selective absorbing layer can be formed bycoating a porous layer with a solution that includes a wavelengthselective absorbing material. The porous layer may be a wavelengthselective scattering layer as discussed herein. The wavelength selectiveabsorbing material includes a dye that enters pores of the porous layer.The solution may include particles that remain on a surface of theporous layer to form a sealant. Forming the wavelength absorbing layermay involve forming a mixture of two or more wavelength selectiveabsorbing materials and depositing the mixture as the wavelengthselective absorbing layer. Alternatively the wavelength selectiveabsorbing layer may comprise two or more layers, wherein a first layerincludes first wavelength selective absorbing material a second layerincludes a second wavelength selective absorbing material.

The wavelength selective absorbing layer 34 may scatter and absorblight. The wavelength selective absorbing layer may scatter more in thevisible range of 400 nm to 700 nm compared to light scattered by thewavelength selective absorbing layer in the near infrared range from 830nm-900 nm, 900 nm-980 nm, and/or 800 nm-1200 nm, for example. Accordingto some implementations, the wavelength selective absorbing layer 34 mayscatter less than about 50%, less than about 40%, less than about 30%,or less than about 25% of light in the visible wavelength range between400 nm and 700 nm The wavelength selective absorbing layer 34 may alsoscatter less than about 50%, less than about 40%, less than about 30%,or less than about 25% of light in the near infrared range from 830nm-900 nm, 900 nm-980 nm, and/or 800 nm-1200 nm.

While FIGS. 1A-1K show respective articles 10 a-10 k as including flatlayers, in various examples, articles 10 a-10 k may assume any suitableshape, periphery, or cross-section, and layers in articles 10 a-10 k mayassume a regular, irregular, or compound curvature, or may assume flator curved geometries in different regions, or otherwise conform to acontour of a substrate beneath the layers or the articles 10 a-10 k. Forexample, articles 10 a-10 k may assume a hemispherical or lenticularshape, or a surface with an irregular contour. In some examples, any ofthe respective wavelength selective layers, for example, the wavelengthselective scattering layer 14, the reflective layer 16, and thewavelength selective absorbing layer 34 may have a shape or thicknessthat varies across a major dimension, for example, by having a spatiallyvariant or periodic pattern that covers at least some area of substrate12 or an underlying layer, from about 1 to about 100% area. Further,while in some examples described above, articles 10 a-10 k of FIGS.1A-1K may include substrate 12, in other examples, articles 10 a-10 kmay not include substrate 12. In some examples, substrate 12 may beflexible. In some examples, articles 10 a-10 k may be flexible and maybe disposed on a flexible substrate. For example, the flexible substratemay include a light source, a sensor, or a photovoltaic cell. In someexamples, articles 10 a-10 k may be continuously flexible or only beflexible in predetermined regions. Thus, example articles according toexamples described with reference to FIGS. 1A-1K may include opticalfilters that block the transmission of visible wavelengths whileallowing the transmission of near-infrared wavelengths. Example articlesand optical filters may be used in example optical systems describedbelow.

FIGS. 2A-2F are conceptual and schematic diagrams of example opticalsystems including an optical filter. FIG. 2A is a conceptual andschematic diagrams of an example optical system including an opticalfilter 10 and a light receiver 40. In examples, the light receiver 40may include a light sensor, camera, CCD, or any other sensor configuredto sense at least a predetermined wavelength region of light. Forexample, light receiver 40 may include a near-infrared sensor. Inexamples, the light receiver 40 may include an object that receiveslight, for example, a solar cell, or an object that at least partiallyabsorbs incident light, for example, a solar heater or any other objectthat receives light. The optical filter 10 may include any of theexample optical filters including a wavelength selective scatteringlayer, as discussed above with reference to FIGS. 1A-1E, or otherexample optical filters described in the disclosure. As shown in FIG.2A, the optical filter 10 may be disposed adjacent the light receiver40. An incident near-infrared ray 42 a may include a near-infraredwavelength, and may be substantially transmitted through the opticalfilter 10 to the light receiver 40. An incident visible ray 44 a mayinclude a visible wavelength and may be substantially reflected orscattered by the optical filter 10, so that the light receiver 40 is atleast partially shielded from the visible ray 44 a, while at leastpartially receiving the near-infrared ray 42 a. In examples, the lightreceiver may be substantially or completely shielded from the visibleray 44 a by the optical filter 10, and may receive substantially all ofnear-infrared ray 42 a.

FIG. 2B is a conceptual and schematic diagrams of an example opticalsystem including the optical filter 10, the light receiver 40, a lightemitter 46, and an object 48. In examples, the light emitter 46 mayinclude a source of any suitable wavelength of light or electromagneticradiation, including visible, near-infrared, or ultraviolet wavelengths.In examples, the light emitter 46 may include a bulb, an incandescentlight source, compact fluorescent light, LEDs, a light guide, or anynatural or artificial sources of light. In examples, the light emitter46 may not generate light, and may only reflect or transmit lightgenerated by a light source. The optical filter 10 may be disposedbetween the light receiver 40 and the object 48. The light emitter maybe disposed on the same side of the optical filter 10 as the lightreceiver 40. A near-infrared ray 42 b transmitted from the light emitter46 may include a near-infrared wavelength, and may be substantiallytransmitted through the optical filter 10 to the object 48. The ray 42 bmay be reflected back by the object 48, and the reflected ray may bemodified by the optical properties of the object 48. The reflected ray42 may be substantially transmitted through the optical filter 10 to thelight receiver 40. An incident visible ray 44 b may include a visiblewavelength and may be substantially reflected or scattered by theoptical filter 10, so that one or both of the light receiver 40 and thelight emitter 46 are at least partially shielded from the visible ray 44a. In examples, the light receiver may be substantially or completelyshielded from the visible ray 44 b by the optical filter 10, and mayreceive substantially all of near-infrared ray 42 b.

FIG. 2C is a conceptual and schematic diagrams of an example opticalsystem including the optical filter 10, the light receiver 40, and theobject 48. The optical filter 10 may be disposed between the lightreceiver 40 and the object 48. An incident near-infrared ray 42 c mayinclude a near-infrared wavelength, and may be substantially transmittedthrough the object 48 and the optical filter 10 to the light receiver40. An incident visible ray 44 c may include a visible wavelength andmay be substantially reflected or scattered by the optical filter 10, sothat the light receiver 40 is at least partially shielded from thevisible ray 44 c, while at least partially receiving the near-infraredray 42 c. In examples, the light receiver 40 may be substantially orcompletely shielded from the visible ray 44 c by the optical filter 10,and may receive substantially all of near-infrared ray 42 c.

FIG. 2D is a conceptual and schematic diagrams of an example opticalsystem including the optical filter 10 and the light receiver 40. Theoptical filter 10 may be disposed adjacent the light receiver 40. Anincident near-infrared ray 42 d may include a near-infrared wavelength,and may be substantially reflected off the optical filter 10 to thelight receiver 40. An incident visible ray 44 d may include a visiblewavelength and may be substantially reflected or scattered by theoptical filter 10, so that the light receiver 40 at least partiallyreceives the visible ray 44 d, while at least partially receiving thenear-infrared ray 42 d.

FIG. 2E is a conceptual and schematic diagrams of an example opticalsystem including the optical filter 10, the light receiver 40, and thelight emitter 46. The optical filter 10 may be disposed between thelight emitter 46 and the light receiver 40. A near-infrared ray 42 etransmitted from the light emitter 46 may include a near-infraredwavelength, and may be substantially transmitted through the opticalfilter 10 to the light receiver 40. An incident visible ray 44 e mayinclude a visible wavelength and may be substantially reflected orscattered by the optical filter 10, so that the light emitter 46 is atleast partially shielded from the visible ray 44 e. In examples, thelight emitter 46 may be substantially or completely shielded from thevisible ray 44 e by the optical filter 10. While the light receiver 40is described in the example optical system of FIG. 2E, in examples, theexample optical system of FIG. 2E may not include a light receiver 40.For example, the example optical system may include the light emitter 46and the optical filter 10, and the optical filter 10 may conceal thelight emitter 46 from visible appearance.

FIG. 2F is a conceptual and schematic diagrams of an example opticalsystem including the optical filter 10, the light receiver 40, a lightemitter 46, and an object 48 f. In examples, the light emitter 46 mayinclude a source of near-infrared wavelengths, for example, anear-infrared bulb or LED. For example, the light emitter may include alaser, a laser diode, or an injection laser. The light receiver 40 mayinclude a sensor or camera sensitive to near-infrared wavelengths. Forexample, the sensor may include a gesture sensor, an optical touchsensor, or a photoelectric sensor such as a sensor that detects adisruption in a continuously sensed light beam. The sensor may includean array or any other group of one kind or different kinds of sensors.The optical filter 10 may be disposed between the light receiver 40 andthe object 48 f. The light emitter 46 may be disposed on the same sideof the optical filter 10 as the light receiver 40. A near-infrared ray42 b transmitted from the light emitter 46 may include a near-infraredwavelength, and may be substantially transmitted through the opticalfilter 10 to the object 48 f. The ray 42 b may be reflected back by theobject 48, and the reflected ray may be modified by the opticalproperties of the object 48 f. The reflected ray 42 may be substantiallytransmitted through the optical filter 10 to the light receiver 40. Insome examples, an incident visible ray 44 b may include a visiblewavelength and may be substantially reflected or scattered by theoptical filter 10, so that one or both of the light receiver 40 and thelight emitter 46 are at least partially shielded from the visible ray 44a. In examples, the light receiver may be substantially or completelyshielded from the visible ray 44 b by the optical filter 10, and mayreceive substantially all of near-infrared ray 42 b.

In some examples, an iris scanning system may include the exampleoptical system of FIG. 2F, for example, where object 48 f includes aneye or iris, and the light receiver 40 is an iris scanner that receivesnear-infrared light emitted by the light emitter 46 and bounced back byobject 48 f. While the light emitter 46 may emit near-infraredwavelengths, the light emitter 46 may also emit visible wavelengths thatmay reveal the presence of the light emitter 46, for example, to a useror viewer. While articles including a wavelength selective layer 16 maybe used to block the transmission of visible wavelengths to camouflagethe light emitter 46 from a visible perception, wavelength selectivereflective layer 16 may allow some visible wavelengths, for example,peaks or bands of visible wavelengths, to be transmitted. In someexamples, optical filter 10 may include a wavelength selective absorbinglayer 34 that blocks the transmission of visible wavelengths transmittedby the wavelength selective reflective layer 16, as discussed above withreference to FIGS. 1i-1k . Thus, in examples, the optical filter 10 mayhave a visible transmittance at 380-800 nm of less than 0.1% and anear-infrared transmittance at 830-900 nm of greater than 50%.Therefore, optical filter 10 may camouflage the light emitter 46 fromvisible perception, even if light emitter 46 emits visible wavelengths,while allowing the iris scanning system to scan the iris by transmittingnear-infrared wavelengths in both directions across optical filter 10.In some examples, the example optical system of FIG. 2F may include morethan one optical filter 10. For example, a first optical filter may bedisposed adjacent the light emitter 46 or the light receiver 40, and asecond optical filter may be disposed adjacent a major surface of object48 f. In some examples, the first and second optical filter respectiveinclude the same or different optical filters. In some examples, theoptical filter 10 may include a retroreflective film or may be disposedacross or along a retroreflective path. In some examples, the object 48f may include a retroreflective film. While an iris scanning system isdescribed above with reference to FIG. 2F, in some examples, the exampleof FIG. 2F may include any biometric or identification system that usesnear-infrared wavelengths for identification, while emitting visiblewavelengths to be concealed or camouflaged from a visible perception.For example, the example system of FIG. 2F may include a fingerprintscanner, a facial recognition system, or a thermal recognition system.

In some embodiments, an article 10 a-10 k illustrated FIGS. 1A through1K may be formed as a component that can be attached to an object, suchas an electronic device that includes one or both of a light emitter anda light receiver. In some embodiments, the article 10 a-10 k can beattached, detached, and reattached to the object. In some embodiments,the object may be retroreflective. The article 10 a-10 k may be a skinor film that can be applied to an object, e.g., such as an electronicdevice or other object, e.g., cellphone, tablet, notebook computer,automobile. The article 10 a-10 k may be decorative and may have text, alogo and/or a design disposed on the article. FIGS. 2G and 2H illustratefront and back views, respectively, of an article 200 configured to fiton object 210. In FIGS. 2G and 2H, object 210 is represented as acellphone and the article 200 is represented as a cellphone cover thatfits on the cellphone. The article 200 includes at least one or moreregions 201 a, 201 b that include wavelength selective layers of theoptical filter. The wavelength selective layers of the optical filterscatter and/or absorb light to camouflage one or both of a light emitterand a light receiver (not shown in FIGS. 2G and 2H). In someembodiments, the wavelength selective layers of the optical filterextend across a majority or substantially all of the article and in someembodiments, portions of the article include the optical filter layersand other portions do not include the optical filter layers may beopaque. The article 200 can include a cling film surface, an adhesiveand/or attachment features to facilitate attaching the article 200 tothe object 210. For example, the cellphone cover 200 shown in FIGS. 2Gand 2H may be attached to the cellphone 210 via attachment featurescomprising a wall 202 b and/or lip 202 a that provide a press-fit wheninstalled on the cellphone. Alternatively, the article 200 may beattached to the object 210 using other types of attachment featuresother than press-fit features, e.g., bolt-on attachment features, suchas a hole configured to accept a bolt or screw that attaches the article200 to the object 210.

In examples, the optical filter 10 may include at least one removable orrepositionable layer, or optical filter 10 as a whole may be removableor repositionable, so that it can be removed or repositioned relative toa substrate underneath or adjacent the optical filter 10. In examples,the periphery of the optical filter 10 may extend beyond the peripheryof one or both the light emitter 46 or the light receiver 40, or thearea of a major surface of the optical filter 10 may be greater orsmaller than a surface area of one or both of the light emitter 46 orthe light receiver 40. In examples, the optical filter 10 may beconfigured to camouflage other components, such as electronics,circuitry, substrates, sensors, transmitters by shielding thosecomponents by the optical filter from a visual perception. In examples,more than one light emitter 46 or light receiver 40, for example, anarray, could be positioned adjacent the optical filter 10. In examples,one or both of the light emitter 46 or the light receiver 40 may berelatively remote from the optical filter 10, for example, at least 1 cmaway, or 10 cm away, or 1 m away or, 10 m away, or 100 m away, or 1 kmaway, or even further remote. While a direct path for light is shown inFIGS. 2A-2F, for example, between one or both of the light emitter 46and the light receiver 40 and the optical filter 10, in examples, lightbetween one or both of the light emitter 46 and the light receiver 40and the optical filter 10 may follow indirect paths, including opticallyguided paths, reflected paths, or paths including optical manipulationincluding refraction or filtering, or paths that travel throughdifferent optical media.

Thus, in examples, the optical filter 10 may be configured to at leastpartially shield the light receiver 40 from visible wavelengths whilesubstantially allowing the light receiver 40 to receive near-infraredwavelengths. For example, optical filter 10 may be configured to concealor camouflage one or more of the light receiver 40, light emitter 46, orobject 48. In examples, the optical filter 10 may be configured tocamouflage one or both of the light receiver 40 or the light emitter 46from a visual perception, for example, by scattering visible wavelengthsas discussed above with reference to FIGS. 2A-2F.

FIGS. 3A-3D are conceptual diagrams of an example system including anexample optical filter and an electronic display displaying a visiblyperceptible pattern and an invisible near-infrared pattern. Sinceimaging sensors such as charge-coupled devices (CCD) detect in thenear-infrared region, it would be possible to produce a sign including avisibly reflective graphic. The sign could conceal an invisible imagethat is detectable by the camera. For example, the image could include apredetermined pattern that encodes a signal or information, such as abar code, a 2D bar code, or a QR code. The physical size of QR codes maylimit the amount of information they may contain. However an invisibleQR code could be physically as large as the sign without cluttering orcompromising the visible graphic. In an example, an electronic display60 may be capable of simultaneously displaying visible and near-infraredpatterns emitted by respective visible and near-infrared light emittersconcealed behind the display 60. The electronic display 60 may becovered with an example optical filter described above with reference toFIGS. 1A-1E. For example, the electronic display 60 may simultaneouslydisplay a pattern 62 that is visible and an invisible near-infraredpattern 64, as shown in FIG. 3B. The pattern 62 may include a relativelysmaller QR code or other indicia with a relatively smaller displayfootprint, while the pattern 64 may include a relatively larger QR codeor other indicia with a relatively larger footprint. The pattern 62 maybe visible as a result of reflection or scattering of visiblewavelengths by the optical filter (not shown). As seen in FIG. 3A, onlypattern 62 may be visibly perceived, and pattern 64 may remain invisibleto visual perception, while being presented with relatively high clarityin near-infrared wavelengths. A camera capable of sensing near-infraredwavelengths may thus sense pattern 64 with sufficient resolution, forexample, with a resolution sufficient to decode information that may becontained in pattern 64. In the example shown in FIG. 3C only apredetermined pattern may be visibly perceptible on display 60, while aninvisible near-infrared pattern only detectable by a near-infraredcamera may be simultaneously displayed on the display 60, as shown inFIG. 3D. Thus, in the respective example systems of 3A and 3B, and 3Cand 3D, an example optical filter may be used to conceal or camouflage asource of a near-infrared pattern while revealing only a predeterminedvisible pattern. In some examples, the invisible near-infrared patterns64 may be used to encode concealed information, while the visiblyperceptible patterns 62 may be used to present visibly perceptibleinformation, or at least information that may be encoded, but is visiblyperceptible as being encoded. For example, pattern 62 may encode a firstset of information, such as a website, while pattern 64 may encode asecond set of information, such as a location of the display 60. Inexamples, the electronic display 60 may display a visible pattern, aninvisible pattern, or both. In examples, the electronic display 60 maydisplay multiple patterns. In examples, the electronic display maydisplay static patterns or dynamic patterns. Thus, example opticalfilters may provide camouflage with high clarity near-infraredtransmission.

FIG. 4 is a flowchart of an example technique. The example technique mayinclude disposing an optical filter 10 adjacent one or both of the lightemitter 46 or the light receiver 40 (52). The optical filter 10 includesa wavelength selective scattering layer, as discussed above withreference to FIGS. 1A-1E and FIGS. 2A-2E. The example technique mayoptionally further include disposing the reflective layer 16 between theoptical filter 10 and one or both of the light emitter 46 or the lightreceiver 40 (54). The optical filter 10 may optionally camouflage one orboth of the light emitter 46 or the light receiver 40 (56). The opticalfilter 10 may optionally at least partially shield one or both of thelight emitter or the light receiver from visible wavelengths (58).

While articles described above may include multilayer films or mayinclude multiple layers, in some examples, one or more layer may beblended into an adjacent layer, or may form a visibly indistinct gradedboundary with an adjacent layer. In some examples, the multilayer filmsor articles may be processed such that no discernible boundaries ormajor surfaces separate one or more layers, and different layers maytransition into adjacent layers. In some examples, a layer may signify apredetermined substantially planar or curved geometric region ratherthan a physically distinct or discrete layer.

Thus, example systems, articles, and techniques according to the presentdisclosure may include example optical articles including examplewavelength selective scattering layers that transmit near-infrared lightwith relatively high clarity while reducing the transmission of visiblewavelengths, for example, by selectively scattering or reflectingvisible wavelengths.

Example articles and techniques according to the disclosure provide willbe illustrated by the following non-limiting embodiments and examples.

EMBODIMENTS

Embodiments of the invention include the following enumerated items.

Item 1. A system comprising:

one or both of a light emitter or a light receiver; and

an optical filter adjacent one or both of the light emitter or the lightreceiver, wherein the optical filter comprises a wavelength selectivescattering layer, wherein the wavelength selective scattering layer hasa near-infrared scattering ratio of less than about 0.9, thenear-infrared scattering ratio being a ratio of an average near-infraredscattering to an average visible scattering, and wherein the wavelengthselective scattering layer has a visible reflective haze ratio ofgreater than about 0.5, the visible reflective haze ratio being a ratioof an average visible diffusive reflectance to an average visible totalreflectance.

Item 2. The system of item 1, wherein the wavelength selectivescattering layer has a near-infrared scattering ratio of less than about0.7.

Item 3. The system of item 1, wherein the wavelength selectivescattering layer has a near-infrared scattering ratio of less than about0.6.

Item 4. The system of any one of items 1 to 3, wherein the wavelengthselective scattering layer has a visible reflective haze ratio ofgreater than about 0.6.

Item 5. The system of any one of items 1 to 4, wherein the wavelengthselective scattering layer has a visible reflective haze ratio ofgreater than about 0.7.

Item 6. The system of any one of items 1 to 5, wherein one or both ofthe light emitter or the light receiver have an operating wavelengthwithin a near-infrared range.

Item 7. The system of any one of items 1 to 6, wherein the wavelengthselective scattering layer transmits less than about 50% of incidentvisible light, and wherein the wavelength selective scattering layertransmits greater than about 50% of incident near-infrared light.

Item 8. The system of any one of items 1 to 7, wherein the wavelengthselective scattering layer scatters greater than about 50% of incidentvisible light.

Item 9. The system of any one of items 1 to 8, wherein the wavelengthselective scattering layer scatters greater than about 50% of incidentvisible light as white light.

Item 10. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 5 μm, and whereinan absolute difference between the first refractive index and the secondrefractive index is less than about 0.1.

Item 11. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 1 μm, and whereinan absolute difference between the first refractive index and the secondrefractive index is less than about 0.2.

Item 12. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 0.5 μm, andwherein an absolute difference between the first refractive index andthe second refractive index is less than about 0.4.

Item 13. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 0.3 μm, andwherein an absolute difference between the first refractive index andthe second refractive index is less than about 0.6.

Item 14. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 0.2 μm, andwherein an absolute difference between the first refractive index andthe second refractive index is less than about 1.8.

Item 15. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 82 of FIG. 15.

Item 16. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 84 of FIG. 15.

Item 17. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 86 of FIG. 15.

Item 18. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises an optical medium have a firstrefractive index, wherein the optical medium comprises a plurality ofparticles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 88 of FIG. 15.

Item 19. The system of any one of items 1 to 18, wherein the wavelengthselective scattering layer has a visible haze of at least 25%.

Item 20. The system of any one of items 1 to 19, wherein the opticalfilter comprises surface optical microstructures.

Item 21. The system of any one of items 1 to 20, wherein the lightemitter comprises a near-infrared LED or a near-infrared laser.

Item 22. The system of any one of items 1 to 21, wherein the lightreceiver comprises a near-infrared camera or a light sensor having anear-infrared receiving band.

Item 23. The system of any one of items 1 to 9, wherein the wavelengthselective scattering layer comprises a binder, a plurality of particles,and a plurality of interconnected voids, wherein a volume fraction ofthe plurality of interconnected voids in the wavelength selectivescattering layer is not less than about 20%, and wherein a weight ratioof the binder to the plurality of the particles is not less than about1:2.

Item 24. The system of any one of items 1 to 23, wherein the opticalfilter comprises a reflective layer.

Item 25. The system of any one of items 1 to 23, wherein the opticalfilter comprises a beaded diffuser layer.

Item 26. The system of any one of items 1 to 25, wherein the opticalfilter is configured to at least partially shield the light receiverfrom visible wavelengths while substantially allowing the light receiverto receive near-infrared wavelengths.

Item 27. The system of any one of items 1 to 26, wherein the opticalfilter is configured to camouflage one or both of the light receiver orthe light emitter from a visual perception.

Item 28. The system of item 27, wherein the optical filter is configuredto at least partially camouflage one or both of the light receiver orthe light emitter from a visual perception by scattering visiblewavelengths.

Item 29. A method comprising:

disposing an optical filter adjacent one or both of a light emitter or alight receiver, wherein the optical filter comprises a wavelengthselective scattering layer, wherein the wavelength selective scatteringlayer has a near-infrared scattering ratio of less than about 0.9, thenear-infrared scattering ratio being a ratio of an average near-infraredscattering to an average visible scattering, and wherein the wavelengthselective scattering layer has a visible reflective haze ratio ofgreater than about 0.5, the visible reflective haze ratio being a ratioof an average visible diffusive reflectance to an average visible totalreflectance.

Item 30. The method of item 29, further comprising disposing areflective layer between the optical filter and one or both of the lightemitter or the light receiver.

Item 31. A method comprising at least partially camouflaging one or bothof the light emitter or the light receiver, the camouflaging comprisingthe method of item 29 or 30.

Item 32. A method comprising at least partially shielding one or both ofthe light emitter or the light receiver from visible wavelengths, theshielding comprising the method of item 29 or 30.

Item 33. The method of any one of items 29 to 32, wherein the wavelengthselective scattering layer scatters greater than about 50% of incidentvisible light.

Item 34. The method of item 33, wherein the wavelength selectivescattering layer scatters greater than about 50% of incident visiblelight as white light.

Item 35. An article comprising an optical filter, wherein the opticalfilter comprises a wavelength selective scattering layer, wherein thewavelength selective scattering layer has a near-infrared scatteringratio of less than about 0.9, the near-infrared scattering ratio being aratio of an average near-infrared scattering to an average visiblescattering, and wherein the wavelength selective scattering layer has avisible reflective haze ratio of greater than about 0.5, the visiblereflective haze ratio being a ratio of an average visible diffusivereflectance to an average visible total reflectance.

Item 36. The article of item 35, wherein the wavelength selectivescattering layer has a near-infrared scattering ratio of less than about0.7.

Item 37. The article of item 36, wherein the wavelength selectivescattering layer has a near-infrared scattering ratio of less than about0.6.

Item 38. The article of any one of items 35 to 37, wherein thewavelength selective scattering layer has a visible reflective hazeratio of greater than about 0.6.

Item 39. The article of any one of items 35 to 37, wherein thewavelength selective scattering layer has a visible reflective hazeratio of greater than about 0.7.

Item 40. The article of any one of items 35 to 39, wherein thewavelength selective scattering layer transmits less than about 50% ofincident visible light, and wherein the wavelength selective scatteringlayer transmits greater than about 50% of incident near-infrared light.

Item 41. The article of any one of items 35 to 40, wherein thewavelength selective scattering layer scatters greater than about 50% ofincident visible light.

Item 42. The article of any one of items 35 to 40, wherein thewavelength selective scattering layer scatters greater than about 50% ofincident visible light as white light.

Item 43. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 5 μm, and whereinan absolute difference between the first refractive index and the secondrefractive index is less than about 0.1.

Item 44. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 1 μm, and whereinan absolute difference between the first refractive index and the secondrefractive index is less than about 0.2.

Item 45. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 0.5 μm, andwherein an absolute difference between the first refractive index andthe second refractive index is less than about 0.4.

Item 46. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 0.3 μm, andwherein an absolute difference between the first refractive index andthe second refractive index is less than about 0.6.

Item 47. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex and an average particle size of less than about 0.2 μm, andwherein an absolute difference between the first refractive index andthe second refractive index is less than about 1.8.

Item 48. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 82 of FIG. 15.

Item 49. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 84 of FIG. 15.

Item 50. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 86 of FIG. 15.

Item 51. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises an optical medium have afirst refractive index, wherein the optical medium comprises a pluralityof particles, wherein the plurality of particles has a second refractiveindex, wherein an average particle size of the plurality of particles,the first refractive index, and the second refractive index are selectedfrom a region under line 88 of FIG. 15.

Item 52. The article of any one of items 35 to 51, wherein thewavelength selective scattering layer has a visible haze of at least25%.

Item 53. The article of any one of items 35 to 52, wherein the opticalfilter comprises surface optical microstructures.

Item 54. The article of any one of items 35 to 42, wherein thewavelength selective scattering layer comprises a binder, a plurality ofparticles, and a plurality of interconnected voids, wherein a volumefraction of the plurality of interconnected voids in the wavelengthselective scattering layer is not less than about 20%, and wherein aweight ratio of the binder to the plurality of the particles is not lessthan about 1:2.

Item 55. The article of any one of items 35 to 54, wherein the opticalfilter comprises a reflective layer.

Item 56. The article of any one of items 35 to 55, wherein the opticalfilter comprises a beaded diffuser layer.

Item 57. The article of any one of items 35 to 56, wherein the opticalfilter is configured to at least partially shield a light receiver fromvisible wavelengths while allowing the light receiver to at leastpartially receive near-infrared wavelengths.

Item 58. The article of any one of items 35 to 56, wherein the opticalfilter is configured to at least partially camouflage one or both of alight receiver and a light emitter from a visual perception.

Item 59. The article of item 58, wherein the optical filter isconfigured to at least partially camouflage one or both of the lightreceiver or the light emitter from a visual perception by at leastpartially scattering visible wavelengths.

Item 60. The article of any one of items 35 to 59, wherein the opticalfilter comprises an ink receptive coating adjacent the wavelengthselective scattering layer.

Item 61. The article of any one of items 35 to 60, wherein the opticalfilter comprises an inked pattern disposed on the ink receptive coating.

Item 62. The article of any one of items 35 to 61, wherein the opticalfilter comprises a sealant layer.

Item 63. The article of any one of items 35 to 62, wherein the opticalfilter comprises a protective coating.

Item 64. The article of any one of items 35 to 63, wherein thewavelength selective scattering layer has a total visible reflectance ofat least 50%.

Item 65. The article of item 64, wherein the wavelength selectivescattering layer has a total visible reflectance of at least 60%.

Item 66. The article of item 65, wherein the wavelength selectivescattering layer has a total visible reflectance of at least 70%.

Item 67. An article comprising an optical filter, wherein the opticalfilter comprises a wavelength selective scattering layer, wherein thewavelength selective scattering layer has an average near-infraredscattering of less than 60%, wherein the wavelength selective scatteringlayer has an average visible scattering of greater than 10%, and whereina difference between the % total visible reflectance and the % diffusevisible reflectance is less than 20.

Item 68. The article of item 67, wherein the wavelength selectivescattering layer has an average near-infrared scattering of less than40%, wherein the wavelength selective scattering layer has an averagevisible scattering of greater than 58%, and wherein the differencebetween the % total visible reflectance and the % diffuse visiblereflectance is less than 18.

Item 69. A system comprising:

one or both of a light emitter or a light receiver; and

an optical filter adjacent one or both of the light emitter or the lightreceiver, wherein the optical filter comprises:

a wavelength selective scattering layer, wherein the wavelengthselective scattering layer is configured to substantially scattervisible wavelengths,

a wavelength selective reflective layer, and

at least one wavelength selective absorbing layer, wherein eachrespective wavelength selective layer is configured to transmitnear-infrared wavelengths.

Item 70. The system of item 69, wherein the optical filter has a visibletransmittance of less than 5% and a near-infrared transmittance ofgreater than 5% for wavelengths greater than 830 nm.

Item 71. The system of item 70, wherein the optical filter has a visibletransmittance of less than 1%.

Item 72. The system of item 70 or 71, wherein the optical filter has anear-infrared transmittance of greater than 10% for wavelengths greaterthan 830 nm.

Item 73. The system of item 72, wherein the optical filter has anear-infrared transmittance of greater than 20% for wavelengths greaterthan 850 nm.

Item 74. The system of item 73, wherein the optical filter has anear-infrared transmittance of greater than 50% for wavelengths greaterthan 870 nm.

Item 75. The system of item 74, wherein the optical filter has anear-infrared transmittance of greater than 50% for wavelengths greaterthan 900 nm.

Item 76. The system of item 75, wherein the optical filter has anaverage near-infrared transmittance of greater than 75% for wavelengthsgreater than 900 nm.

Item 77. The system of any one of items 69 to 76, wherein the wavelengthselective scattering layer has a near-infrared scattering ratio of lessthan about 0.9, the near-infrared scattering ratio being a ratio of anaverage near-infrared scattering to an average visible scattering, andwherein the wavelength selective scattering layer has a visiblereflective haze ratio of greater than about 0.5, the visible reflectivehaze ratio being a ratio of an average visible diffusive reflectance toan average visible total reflectance.

Item 78. The system of any one of items 69 to 76, wherein the wavelengthselective scattering layer has an average near-infrared scattering ofless than 60%, wherein the wavelength selective scattering layer has anaverage visible scattering of greater than 10%, and wherein a differencebetween the % total visible reflectance and the % diffuse visiblereflectance is less than 20.

Item 79. The system of any one of items 69 to 76, wherein the wavelengthselective scattering layer transmits less than about 50% of incidentvisible light, and wherein the wavelength selective scattering layertransmits greater than about 50% of incident near-infrared light.

Item 80. The system of any one of items 69 to 79, wherein the wavelengthselective scattering layer comprises a binder, a plurality of particles,and a plurality of interconnected voids, wherein a volume fraction ofthe plurality of interconnected voids in the wavelength selectivescattering layer is not less than about 20%, and wherein a weight ratioof the binder to the plurality of the particles is not less than about1:2.

Item 81. The system of any one of items 69 to 80, wherein the lightemitter comprises a near-infrared LED or a near-infrared laser.

Item 82. The system of any one of items 69 to 81, wherein the lightreceiver comprises a near-infrared camera or a light sensor having anear-infrared receiving band.

Item 83. The system of any one of items 69 to 82, wherein the reflectivelayer comprises a multilayer optical film.

Item 84. The system of any one of items 69 to 83, wherein the reflectivelayer comprises a wavelength selective interference filter.

Item 85. The system of any one of items 69 to 84, wherein the opticalfilter is disposed on a substrate layer.

Item 86. The system of any one of items 69 to 85, wherein the opticalfilter is configured to at least partially shield the light receiverfrom visible wavelengths while substantially allowing the light receiverto receive near-infrared wavelengths.

Item 87. The system of any one of items 69 to 86, wherein the opticalfilter is configured to camouflage one or both of the light receiver orthe light emitter from a visual perception.

Item 88. The system of item 87, wherein the optical filter is configuredto at least partially camouflage one or both of the light receiver orthe light emitter from a visual perception by scattering visiblewavelengths.

Item 89. The system of any one of items 69 to 88, wherein the wavelengthselective absorbing layer is between the wavelength selective scatteringlayer and the wavelength selective reflective layer, and wherein thewavelength selective absorbing layer is configured to reduce a totalvisible reflectance of the optical filter by a predetermined magnitudewithout substantially reducing a total near-infrared transmittance.

Item 90. The system of any one of items 69 to 88, wherein the wavelengthselective reflective layer is between the wavelength selectivescattering layer and the wavelength selective absorbing layer, andwherein the wavelength selective absorbing layer is configured to reducea total visible reflectance uniformly over an area of a major surface ofthe optical filter without substantially reducing a total near-infraredtransmittance.

Item 91. An article comprising an optical filter, wherein the opticalfilter comprises:

a wavelength selective scattering layer, wherein the wavelengthselective scattering layer is configured to substantially scattervisible wavelengths,

a wavelength selective reflective layer, and

at least one wavelength selective absorbing layer, wherein eachrespective wavelength selective layer is configured to transmitnear-infrared wavelengths.

Item 92. The article of item 91, wherein the optical filter has avisible transmittance of less than 5% and a near-infrared transmittanceof greater than 5% for wavelengths greater than 830 nm.

Item 93. The article of item 92, wherein the optical filter has avisible transmittance of less than 1%.

Item 94. The article of items 92 or 93, wherein the optical filter has anear-infrared transmittance of greater than 10% for wavelengths greaterthan 830 nm.

Item 95. The article of item 94, wherein the optical filter has anear-infrared transmittance of greater than 20% for wavelengths greaterthan 850 nm.

Item 96. The article of item 95, wherein the optical filter has anear-infrared transmittance of greater than 50% for wavelengths greaterthan 870 nm.

Item 97. The article of item 96, wherein the optical filter has anear-infrared transmittance of greater than 50% for wavelengths greaterthan 900 nm.

Item 98. The article of item 97, wherein the optical filter has anaverage near-infrared transmittance of greater than 75% for wavelengthsgreater than 900 nm.

Item 99. The article of any one of items 91 to 98, wherein thewavelength selective scattering layer has a near-infrared scatteringratio of less than about 0.9, the near-infrared scattering ratio being aratio of an average near-infrared scattering to an average visiblescattering, and wherein the wavelength selective scattering layer has avisible reflective haze ratio of greater than about 0.5, the visiblereflective haze ratio being a ratio of an average visible diffusivereflectance to an average visible total reflectance.

Item 100. The article of any one of items 91 to 99, wherein thewavelength selective scattering layer has an average near-infraredscattering of less than 60%, wherein the wavelength selective scatteringlayer has an average visible scattering of greater than 10%, and whereina difference between the % total visible reflectance and the % diffusevisible reflectance is less than 20.

Item 101. The article of any one of items 91 to 100, wherein thewavelength selective scattering layer transmits less than about 50% ofincident visible light, and wherein the wavelength selective scatteringlayer transmits greater than about 50% of incident near-infrared light.

Item 102. The article of any one of items 91 to 101, wherein thewavelength selective scattering layer comprises a binder, a plurality ofparticles, and a plurality of interconnected voids, wherein a volumefraction of the plurality of interconnected voids in the wavelengthselective scattering layer is not less than about 20%, and wherein aweight ratio of the binder to the plurality of the particles is not lessthan about 1:2.

Item 103. The article of any one of items 91 to 102, wherein thereflective layer comprises a multilayer optical film.

Item 104. The article of any one of items 91 to 103, wherein thereflective layer comprises a wavelength selective interference filter.

Item 105. The article of any one of items 91 to 104, wherein the opticalfilter is disposed on a substrate layer.

Item 106. The article of any one of items 91 to 105, wherein the opticalfilter comprises an ink receptive coating adjacent the wavelengthselective scattering layer.

Item 107. The article of item 106, wherein the optical filter comprisesan inked pattern disposed on the ink receptive coating.

Item 108. The article of any one of items 91 to 107, wherein the opticalfilter comprises a sealant layer.

Item 109. The article of any one of items 91 to 107, wherein the opticalfilter comprises a protective coating.

Item 110. The article of any one of items 91 to 109, wherein thewavelength selective absorbing layer is between the wavelength selectivescattering layer and the wavelength selective reflective layer, andwherein the wavelength selective absorbing layer is configured to reducea total visible reflectance of the optical filter by a predeterminedmagnitude without substantially reducing a total near-infraredtransmittance.

Item 111. The article of any one of items 91 to 109, wherein thewavelength selective reflective layer is between the wavelengthselective scattering layer and the wavelength selective absorbing layer,and wherein the wavelength selective absorbing layer is configured toreduce a total visible reflectance uniformly over an area of a majorsurface of the optical filter without substantially reducing a totalnear-infrared transmittance.

Item 112. A system comprising:

one or both of a light emitter or a light receiver; and

an optical filter adjacent one or both of the light emitter or the lightreceiver, wherein the optical filter comprises:

a wavelength selective reflective layer, and

at least one wavelength selective absorbing layer, wherein eachrespective wavelength selective layer is configured to transmitnear-infrared wavelengths, and wherein the optical filter has a visibletransmittance at 380-800 nm of less than 0.1% and a near-infraredtransmittance at 830-900 nm of greater than 50%.

Item 113. The system of item 112, wherein the optical filter has avisible transmittance at 380-800 nm of less than 0.01% and anear-infrared transmittance at 830-900 nm of greater than 75%.

Item 114. The system of items 112 or 113, wherein the light emittercomprises a near-infrared LED or a near-infrared laser.

Item 115. The system of any one of items 112 to 114, wherein the lightreceiver comprises a near-infrared camera or a light sensor having anear-infrared receiving band.

Item 116. The system of any one of items 112 to 115, wherein the lightreceiver comprises an iris scanning system.

Item 117. A system comprising an iris-based identification system,comprising the system of any one of items 112 to 116.

Item 118. The system of any one of items 112 to 117, wherein thereflective layer comprises a wavelength selective interference filter.

Item 119. The system of any one of items 112 to 118, wherein thereflective layer comprises a multilayer optical film.

Item 120. The system of any one of items 112 to 119, wherein the opticalfilter is disposed on a substrate layer.

Item 121. The system of any one of items 112 to 120, wherein the opticalfilter is configured to at least partially shield the light receiverfrom visible wavelengths while substantially allowing the light receiverto receive near-infrared wavelengths.

Item 122. The system of any one of items 112 to 121, wherein the opticalfilter is configured to camouflage one or both of the light receiver orthe light emitter from a visual perception.

Item 123. The system of any one of items 112 to 122, wherein the atleast one wavelength selective absorbing layer comprises a firstwavelength selective absorbing layer and a second wavelength selectiveabsorbing layer, and wherein the wavelength selective reflective layeris between the first wavelength selective absorbing layer and the secondwavelength selective absorbing layer.

Item 124. An article comprising an optical filter comprising:

a wavelength selective reflective layer, and

at least one wavelength selective absorbing layer, wherein eachrespective wavelength selective layer is configured to transmitnear-infrared wavelengths, and wherein the optical filter has a visibletransmittance at 380-800 nm of less than 0.1% and a near-infraredtransmittance at 830-900 nm of greater than 50%.

Item 125. The article of item 124, wherein the optical filter has avisible transmittance at 380-800 nm of less than 0.01% and anear-infrared transmittance at 830-900 nm of greater than 75%.

Item 126. The article of items 124 or 125, wherein the reflective layercomprises a wavelength selective interference filter.

Item 127. The article of any one of items 124 to 126, wherein thereflective layer comprises a multilayer optical film.

Item 128. The article of any one of items 124 to 127, wherein theoptical filter is disposed on a substrate layer.

Item 129. The article of any one of items 124 to 128, wherein the atleast one wavelength selective absorbing layer comprises a firstwavelength selective absorbing layer and a second wavelength selectiveabsorbing layer, and wherein the wavelength selective reflective layeris between the first wavelength selective absorbing layer and the secondwavelength selective absorbing layer.

Item 130. The article of any one of items 124 to 130, wherein theoptical filter comprises a sealant layer.

Item 131. The article of any one of items 124 to 130, wherein theoptical filter comprises a protective coating.

Item 132. The article of any one of items 124 to 131, wherein thewavelength selective absorbing layer comprises one or both of awavelength selective dye or a wavelength selective pigment.

Item 133. The system of any one of items 69 to 90, wherein thewavelength selective absorbing layer comprises one or both of awavelength selective dye or a wavelength selective pigment.

Item 134. The article of any one of items 91 to 111, wherein thewavelength selective absorbing layer comprises one or both of awavelength selective dye or a wavelength selective pigment.

Item 135. The system of any one of items 112 to 123, wherein thewavelength selective absorbing layer comprises one or both of awavelength selective dye or a wavelength selective pigment.

Item 136. An article that includes an optical filter, the optical filtercomprising:

a wavelength selective reflective layer; and

at least one wavelength selective absorbing layer, wherein the opticalfilter has average visible transmittance for wavelengths between 400nm-700 nm less than about 30% and average near infrared transmittancefor wavelengths between 830 nm-900 nm greater than about 30%.

Item 137. The article of item 136, wherein average visible transmittancefor wavelengths between 400 nm-700 nm is less than about 5%.

Item 138. The article of any of items 136 through 137, wherein averagenear infrared transmittance for wavelengths between 830 nm-900 nm isgreater than about 50%.

Item 139. The article of any of items 136 through 138, wherein thewavelength selective reflective layer comprises an interference filter.

Item 140. The article of any of items 136 through 138, wherein thewavelength selective reflective layer comprises a multilayer opticalfilm.

Item 141. The article of any of items 136 through 138, wherein thewavelength selective reflective layer comprises a reflective polarizer.

Item 142. The article of any of items 136 through 141, wherein thewavelength selective absorbing layer comprises one or both of awavelength selective dye and a wavelength selective pigment.

Item 143. The article of item 142, wherein the wavelength selectiveabsorbing layer comprises a porous layer and one or both of thewavelength selective dye and the wavelength selective pigment isdisposed within pores of the porous layer.

Item 144. The article of item 142, wherein one or both of the wavelengthselective dye and the wavelength selective pigment absorbs light in afirst spectral range and re-emits light in a different, second spectralrange.

Item 145. The article of any of items 136 through 144, wherein the atleast one wavelength selective absorbing layer comprises a firstwavelength selective absorbing layer and a second wavelength selectiveabsorbing layer, and the wavelength selective reflective layer isbetween the first wavelength selective absorbing layer and the secondwavelength selective absorbing layer.

Item 146. The article of item 145, wherein the first wavelengthselective absorbing layer has different optical characteristics than thesecond wavelength selective absorbing layer.

Item 147. The article of item 146, wherein:

the first wavelength selective absorbing layer comprises one or both ofa black dye and a black pigment; and

the second wavelength selective absorbing layer comprises one or both ofa color dye and a color pigment.

Item 148. The article of item 147, wherein the color dye or the colorpigment comprises one or more of cyan, magenta, and yellow components.

Item 149. The article of any of items 136 through 148, wherein theoptical filter comprises a sealant layer.

Item 150. The article of item 149, wherein the wavelength selectiveabsorbing layer is disposed on the sealant layer.

Item 151. The article of item 150, wherein the wavelength selectiveabsorbing layer is coated or printed on the sealant layer.

Item 152. The article of any of items 136 through 151, wherein thewavelength selective absorbing layer scatters less than about 50% ofwavelengths between 400 nm-700 nm and scatters less than about 50% ofwavelengths between 830 nm and 900 nm.

Item 153. The article of item 52, wherein the wavelength selectiveabsorbing layer scatters less than about 30% of wavelengths between 400nm-700 nm and scatters less than about 30% of wavelengths between 830 nmand 900 nm.

Item 154. The article of any of items 136 through 153, wherein thewavelength selective absorbing layer scatters more light in visiblewavelengths between 400 nm-700 nm compared to light scattered in nearinfrared wavelengths between 830 nm and 900 nm.

Item 155. The article of any of items 136 through 153, wherein thewavelength selective absorbing layer scatters more light in visiblewavelengths between 400 nm-700 nm compared to light scattered in nearinfrared wavelengths between 800 nm and 1200 nm.

Item 156. The article of any of items 136 through 153, wherein thewavelength selective absorbing layer scatters more light in visiblewavelengths between 400 nm-700 nm compared to light scattered in nearinfrared wavelengths between 900 nm and 980 nm.

Item 157. The article of any of items 136 through 156, wherein thewavelength selective absorbing layer is a printed layer.

Item 158. The article of any of items 136 through 156, wherein thewavelength selective absorbing layer comprises a coating on anotherlayer.

Item 159. The article of any of items 136 through 158, furthercomprising a wavelength selective scattering layer.

Item 160. The article of item 159, wherein the wavelength selectiveabsorbing layer is disposed on the wavelength selective scatteringlayer.

Item 161. The article of item 159, wherein the wavelength selectiveabsorbing layer is coated on the wavelength selective scattering layer.

Item 162. The article of item 159, wherein the wavelength selectiveabsorbing layer is printed on the wavelength selective scattering layer.

Item 163. The article of any of items 136 through 162, wherein theoptical filter is flexible.

Item 164. The article of any of items 136 through 163, wherein theoptical filter has a three dimensional shape.

Item 165. The article of any of items 136 through 164, furthercomprising a substrate.

Item 166. The article of item 165, wherein the substrate comprises atleast one of glass and a polymer.

Item 167. The article of item 165, wherein the substrate is a threedimensional substrate.

Item 168. The article of any of items 136 through 167, wherein thearticle has a three dimensional shape and includes one or moreattachment features configured to attach the article to an object.

Item 169. The article of item 168, wherein the attachment featuresinclude one or more press-fit attachment features.

Item 170. The article of item 168, wherein the object is an electroniccomponent.

Item 171. The article of any of items 136 through 170, wherein theoptical filter has visible transmittance for all wavelengths between 400nm-700 nm less than about 30% and near infrared transmittance for allwavelengths between 830 nm to 900 nm greater than about 30%.

Item 172. The article of any of items 136 through 170, wherein theoptical filter has average near infrared transmittance for wavelengthsbetween 800 nm-1200 nm greater than about 30%.

Item 173. The article of any of items 136 through 170, wherein theoptical filter has near infrared transmittance for all wavelengthsbetween 800 nm-1200 nm greater than about 30%.

Item 174. The article of any of items 136 through 173, wherein visibletransmittance of the optical filter at normal incidence is less thanvisible transmittance of the optical filter at an oblique angle.

Item 175. The article of any of items 136 through 173, wherein visibletransmittance of the optical filter at an oblique angle is less thanvisible transmittance of the optical filter at normal incidence.

Item 176. The article of any of items 1 through 173, wherein visibletransmittance of the optical filter at an oblique angle between 0 and 60degrees is less than visible transmittance of the optical filter atnormal incidence.

Item 177. An article that includes an optical filter, the optical filtercomprising:

a wavelength selective reflective layer; and

at least one wavelength selective absorbing layer, wherein the opticalfilter has average visible transmittance for wavelengths between 400nm-700 nm less than about 30% and average near infrared transmittancefor wavelengths between 900 nm-980 nm greater than about 30%.

Item 178. A printed article that includes an optical filter, the opticalfilter comprising:

a wavelength selective reflective layer; and

at least one printed wavelength selective absorbing layer, wherein theoptical filter has average visible transmittance for wavelengths between400 nm-700 nm of less than about 30% and average near infraredtransmittance for wavelengths between 830 nm-900 nm greater than about30%.

Item 179. The printed article of item 178, wherein the optical filterhas visible transmittance for all wavelengths between 400 nm-700 nm ofless than about 30% and near infrared transmittance for all wavelengthsbetween 830 nm-900 nm greater than about 30%.

Item 180. The printed article of item 178, wherein the optical filterhas average visible transmittance for wavelengths between 400 nm-700 nmof less than about 30% and average near infrared transmittance for allwavelengths between 800 nm-1200 nm greater than about 30%.

Item 181. A printed article that includes an optical filter, the opticalfilter comprising:

-   -   a wavelength selective reflective layer; and    -   at least one printed wavelength selective absorbing layer,        wherein the optical filter has average visible transmittance for        wavelengths between 400 nm-700 nm of less than about 30% and        average near infrared transmittance for wavelengths between 900        nm-980 nm greater than about 30%.

Item 182. A printed article that includes an optical filter, the opticalfilter comprising:

-   -   a wavelength selective reflective layer; and    -   at least one printed wavelength selective absorbing layer,        wherein the optical filter has a visible transmittance for all        wavelengths between 400 nm-700 nm of less than about 30% and a        near infrared transmittance for all wavelengths at 900 nm-980 nm        greater than about 30%.

Item 183. A system comprising:

an object; and

an optical filter adjacent the object, the optical filter comprising:

a wavelength selective reflective layer; and

at least one wavelength selective absorbing layer, wherein the opticalfilter has average visible transmittance for wavelengths between 400nm-700 nm less than about 30% and average near infrared transmittancefor wavelengths between 830 nm-900 nm greater than about 30%.

Item 184. The system of item 183, wherein average visible transmittanceis less than about 1%.

Item 185. The system of any of items 183 through 184, wherein theaverage near infrared transmittance is greater than about 75%.

Item 186. The system of any of items 183 through 185, wherein thewavelength selective absorbing layer scatters less than 50% ofwavelengths between 400 nm-700 nm and less than 50% of wavelengthsbetween 830 nm and 900 nm.

Item 187. The system of any of items 183 through 186, wherein thewavelength selective absorbing layer scatters less than about 25% ofwavelengths between 400 nm-700 nm and less 25% of wavelengths between830 nm and 900 nm.

Item 188. The system of any of items 183 through 187, wherein thewavelength selective absorbing layer scatters more light in visiblewavelengths between 400 nm-700 nm compared to light scattered in nearinfrared wavelengths between 830 nm and 900 nm.

Item 189. The system of any of items 183 through 188, wherein the lightemitter comprises a near-infrared LED or a near-infrared laser.

Item 190. The system of any of items 183 through 188, wherein the lightreceiver comprises a near-infrared camera or a light sensor having anear-infrared receiving band.

Item 191. The system of any of items 183 through 188, wherein the lightreceiver comprises an iris scanning system.

Item 192. The system of any of items 183 through 191, wherein theoptical filter is disposed on a substrate layer.

Item 193. The system of any of items 183 through 192, wherein theoptical filter is configured to at least partially shield the lightreceiver from visible wavelengths while substantially allowing the lightreceiver to receive near-infrared wavelengths.

Item 194. The system any of items 183 through 193, wherein the opticalfilter is configured to camouflage one or both of the light receiver andthe light emitter from visual perception.

Item 195. The system of any of items 183 through 194 wherein the opticalfilter further comprises a wavelength selective scattering layer.

Item 196. The system of any of items 183 through 195, wherein the atleast one light emitter and light receiver are components of anelectronic device and the optical filter is a component of an articlethat has a three dimensional shape and includes one or more attachmentfeatures configured to attach the article including the optical filterto the electronic device.

Item 197. The system of any of claims 183 through 196, wherein theobject comprises one or more of a light emitter and a light receiver.

Item 198. The system of any of claims 183 through 197, wherein theobject is retroreflective.

Item 199. An article that includes an optical filter, the optical filtercomprising:

a wavelength selective reflective layer; and

at least one wavelength selective absorbing layer having average visibleabsorption for wavelengths between 400 nm-700 nm greater than about 30%,wherein the optical filter has average near infrared transmittance forwavelengths between 830 nm-900 nm greater than about 30%.

Item 200. The article of item 199, wherein:

the at least one wavelength selective absorbing layer has visibleabsorption for all wavelengths between 400 nm-700 nm greater than about30%, wherein the optical filter has near infrared transmittance for allwavelengths between 830 nm-900 nm greater than about 30%.

Item 201. The article of item 199, wherein average visible absorptionfor wavelengths between 400 nm-700 nm is greater than about 50%.

Item 202. The article of any of items 199 through 201, wherein thewavelength selective absorbing layer scatters less than 50% ofwavelengths between 400 nm-700 nm and less than 50% of wavelengthsbetween 830 nm and 900 nm.

Item 203. The article of any of items 199 through 201, wherein thewavelength selective absorbing layer scatters more light in visiblewavelengths between 400 nm-700 nm compared to light scattered in nearinfrared wavelengths between 830 nm and 900 nm.

Item 204. The article of any of items 199 through 201, wherein thewavelength selective reflective layer has average near infraredtransmittance for wavelengths between 830 nm-900 nm greater than about50%.

Item 205. The article of any of items 199 through 204, furthercomprising a wavelength selective scattering layer.

Item 206. An article that includes an optical filter, the optical filtercomprising:

a wavelength selective reflective layer; and

at least one wavelength selective absorbing layer having average visibleabsorption for wavelengths between 400 nm-700 nm greater than about 30%,wherein the optical filter has average near infrared transmittance forwavelengths between 900 nm-980 nm greater than about 30%.

Item 207. An article that includes an optical filter, the optical filtercomprising:

a wavelength selective reflective layer; and

at least one wavelength selective absorbing layer having average visibleabsorption for wavelengths between 400 nm-700 nm greater than about 30%,wherein the optical filter has average near infrared transmittance forwavelengths between 800 nm-1200 nm greater than about 30%.

Item 208. A system comprising:

an object; and

an optical filter adjacent the object, the optical filter comprising:

a wavelength selective reflective layer having average near infraredtransmittance for wavelengths between 830 nm-900 nm greater than about30%; and

at least one wavelength selective absorbing layer having average visibleabsorption at 400 nm-700 nm greater than about 30% and average nearinfrared transmittance for wavelengths between 830 nm-900 nm greaterthan about 30%.

Item 209. The system of item 208, wherein the light emitter comprises anear-infrared LED or a near-infrared laser.

Item 210. The system of item 208, wherein the light receiver comprises anear-infrared camera or a light sensor having a near-infrared receivingband.

Item 211. The system of item 208, wherein the light receiver comprisesan iris scanning system.

Item 212. The system of item any of items 208 through 211, wherein theoptical filter is configured to at least partially shield the lightreceiver from visible wavelengths while substantially allowing the lightreceiver to receive near-infrared wavelengths.

Item 213. The system of any of items 208 through 212, wherein theoptical filter is configured to camouflage one or both of the lightreceiver and the light emitter from visual perception.

Item 214. The system of any of claims 208 through 213, wherein theobject comprises one or more of a light emitter and a light receiver.

Item 215. The system of any of claims 208 through 214, wherein theobject is retroreflective.

Item 216. An article that includes an optical filter, the optical filtercomprising:

a wavelength selective scattering layer comprising at least one of a dyeand a pigment, the wavelength selective scattering layer configured toscatter visible wavelengths between 400 nm-700 nm and to transmitnear-infrared wavelengths between 830 nm-900 nm; and a wavelengthselective reflective layer configured to transmit near-infraredwavelengths between 830 nm-900 nm.

Item 217. The article of item 216, wherein the wavelength selectivescattering layer comprises a coating that includes at least one of thedye and the pigment.

Item 218. The article of item 216, wherein the coating contains morethan about 11% solids.

Item 219. The article of item 216, wherein the coating contains morethan about 12% solids.

Item 220. The article of item 216, wherein the coating includes morethan about 13% solids.

Item 221. The article of item 216, wherein the coating includes morethan about 14% solids.

Item 222. A method of making an optical filter comprising:

forming a wavelength selective absorbing layer and a wavelengthselective reflective layer, wherein the optical filter has averagevisible transmittance for wavelengths between 400 nm-700 nm of less thanabout 30% and average near infrared transmittance for wavelengthsbetween 830 nm to 900 nm greater than about 30%.

Item 223. The method of item 222, wherein forming the wavelengthselective absorbing layer and the wavelength selective reflective layercomprises forming the wavelength selective absorbing layer on thewavelength selective reflective layer or forming the wavelengthselective reflective layer on the wavelength selective absorbing layer.

Item 224. The method of item 222, wherein forming the wavelengthselective absorbing layer and the wavelength selective reflective layercomprises forming a single combined layer.

Item 225. The method of any of items 222 through 224, wherein formingthe wavelength selective absorbing layer comprises printing or coating awavelength selective absorbing material.

Item 226. The method of item 225, wherein printing or coating thewavelength selective absorbing layer comprises printing or coating asolution that includes two or more of a wavelength selective absorbingmaterial, a wavelength selective scattering material, a wavelengthselective reflective material, and a sealant material.

Item 227. The method of item 222, wherein forming the wavelengthselective absorbing layer comprises coating a porous layer with asolution that includes a wavelength selective absorbing material.

Item 228. The method of item 227, wherein the porous layer is awavelength selective scattering layer.

Item 229. The method of item 227, wherein the wavelength selectiveabsorbing material comprises a dye that enters pores of the porous layerand the solution includes particles that remain on a surface of theporous layer to form a sealant.

Item 230. The method of item 222, wherein forming the wavelengthabsorbing layer comprises forming a mixture of two or more wavelengthselective absorbing materials together and depositing the mixture as thewavelength selective absorbing layer.

Item 231. The method of item 222, wherein forming the wavelengthabsorbing layer comprises:

forming a first wavelength selective absorbing layer comprising a firstwavelength selective absorbing material; and

forming a second wavelength selective absorbing layer comprising asecond wavelength selective absorbing material.

EXAMPLES Example 1

Optical properties for various sample optical films were determined.Sample optical films S01 to S34 were prepared as described below. Thevisible scattering, the near-infrared scattering, total visiblereflectance, and diffuse visible reflectance were measured for each ofsamples S01 to S33, using a spectrometer (Lambda 900, PerkinElmer) withintegrating spheres to capture diffuse and specular reflectance. Theresults are presented in TABLE 1. The presented reflectance valuesinclude SPIN (specular included, or total) and SPEX (specular excluded,or diffuse) reflectances. The sensitivity of a proximity sensor coveredwith the respective sample films was determined, and categorized as oneof “Not Working,” “Functional,” “Good,” and “Excellent.” Thetransmittance, haze, and clarity was determined for samples S01 to S34,using a haze meter (Haze-gard Plus, BYK-Gardner). The results arepresented in TABLE 2.

Samples S01 to S03 were ULI films, with sample S02 including a high hazehigh clarity ULI film. Sample S01 was prepared by combining SilquestA-174 75 nm silane particles (Momentive) with pentaerythritoltriacrylate monomer (SR444, Sartomer) in a 60% wt ratio, and 2.5% ofIrgacure 184 (Ciba Specialty Chemicals Company, High Point N.C), toarrive at a coating thickness of 10 μm. Sample S04 included a film ofTiO2 nanoparticles and silicone microparticles. Sample S04 was preparedby mixing 19.13 g of M1192 (Miwon), 3.38 g of CN9018 (Sartomer), 2.5 gof Tospearl 145 (Momentive), 12.5 g of SR415 (Sartomer), 12.5 g of 42.3wt % TiO2 (UV-TITAN L-530, Sachtleben) in IBOA, 25 g ofmethylethylketone, and 0.5 g of photoinitiator TPO-L (BASF), and coatingthe formulation with a #8 Mayer bar. Sample S05 was a film having amicroreplicated surface structure (FIG. 9). Sample S6 included 3 μmpolystyrene beads coated on ESR2 film (Enhanced Specular Reflector, 3M)for 10 micron dry thickness, with pentaerythritol triacrylate binder(SR444, Sartomer) and isopropyl alcohol solvent. Sample S07 included anon-woven material (a bottom diffuser disassembled from a Sony TV model40W600B). Sample S08 included a TiO2 coated PET film, SH2FGST FasaraFilm (3M). Samples S09 and S10 are bulk diffusers with different hazevalues. Sample S09 included PATTCLR0 frosted acrylate sheet (ePlastics,San Diego, Calif.). Sample S10 included a diffuser plate from a TCL TV(model 40FD2700). Sample S11 was a bottom diffuser sheet from an iPad(first generation, Apple) backlight. Sample S12 included a film ofplastic including dispersed TiO2 (plastic 6″×8″ pint size seal top foodbag with white write-on block, from Elkay Plastics, Bensenville, Ill.).Sample S13 includes white paper (HAmmermill Copy Plus multipurposeprinter paper). Sample S14 includes a film having a microreplicatedsurface structure (iPhone 6 backlight). Samples S15 to S22 include filmsof ULI material. Sample S23 includes sample S04 folded over itself.Sample S24 includes sample S03 folded over itself. Sample S25 includessample S15 folded over itself. Sample S26 includes sample S16 foldedover itself. Sample S27 includes sample S17 folded over itself. SampleS28 includes sample S18 folded over itself. Sample S29 includes sampleS19 folded over itself. Sample S30 includes sample S20 folded overitself. Sample S31 includes sample S21 folded over itself. Sample S32includes sample S2 folded over itself. Sample S33 includes sample S22folded over itself.

TABLE 1 Scattering Visible NIR Sensitivity of (400 nm- (800 nm- VisibleReflection proximity sensor 700 nm) 1200 nm) SPIN SPEX covered withSample scattering scattering Ratio (total) (diffuse) Ratio sample filmS01 86.01 32.19 0.37 55.51 51.60 0.93 Good S02 60.62 7.96 0.13 40.4739.85 0.98 Excellent S03 24.18 4.47 0.19 24.47 21.01 0.86 Excellent S0486.21 57.50 0.67 37.82 35.71 0.94 Functional S05 8.40 5.00 0.60 7.106.15 0.87 Excellent S06 98.29 98.43 1.00 55.35 55.42 1.00 Not WorkingS07 99.05 98.74 1.00 43.43 43.95 1.01 Not Working S08 97.66 90.26 0.9251.62 52.29 1.01 Not Working S09 87.62 88.55 1.01 7.48 6.82 0.91 NotWorking S10 99.50 99.18 1.00 19.77 19.36 0.98 Not Working S11 91.8787.81 0.96 14.67 14.35 0.98 Not Working S12 98.94 93.61 0.95 45.41 45.871.01 Not Working S13 99.46 99.61 1.00 76.89 77.67 1.01 Not Working S1489.00 88.00 0.99 8.87 8.63 0.97 Not Working S15 1.99 0.70 0.35 10.681.64 0.15 Excellent S16 2.39 0.49 0.21 9.69 0.66 0.07 Excellent S17 2.480.43 0.17 9.15 0.53 0.06 Excellent S18 30.19 4.54 0.15 25.47 21.87 0.86Excellent S19 16.42 3.18 0.19 14.01 7.25 0.52 Excellent S20 1.91 1.010.53 10.32 0.96 0.09 Excellent S21 37.35 36.43 0.98 15.20 9.52 0.63 GoodS22 99.70 98.87 0.99 56.51 56.43 1.00 Not Working S23 97.58 83.67 0.8654.77 49.05 0.90 Not Working S24 53.02 11.18 0.21 39.14 28.48 0.73Excellent S25 6.62 2.42 0.37 18.64 2.83 0.15 Excellent S26 4.45 1.900.43 17.61 1.38 0.08 Excellent S27 4.46 1.98 0.44 17.13 1.34 0.08Excellent S28 51.82 9.79 0.19 39.49 29.00 0.73 Excellent S29 31.93 7.350.23 24.97 12.60 0.50 Excellent S30 5.52 2.85 0.52 18.27 1.44 0.08Excellent S31 65.01 61.48 0.95 27.83 16.51 0.59 Functional S32 81.0417.08 0.21 56.29 50.05 0.89 Good S33 99.68 99.53 1.00 71.10 66.58 0.94Not Working

TABLE 2 BYK Haze Gard Values (Visible) Sample Transmission Haze ClarityS01 52.00 90.00 80.00 S02 67.70 64.90 99.40 S03 83.10 28.70 99.20 S0469.50 90.70 91.60 S05 95.90 53.50 97.50 S06 1.06 99.50 35.60 S07 65.60102.00 23.00 S08 58.20 101.00 65.20 S09 92.50 94.50 8.80 S10 65.80102.00 6.20 S11 93.60 95.70 12.10 S12 61.60 102.00 22.40 S13 24.20102.00 4.60 S14 94.00 95.40 7.00 S15 92.20 2.13 99.60 S16 93.30 1.6199.60 S17 93.60 1.03 100.00 S18 83.40 28.80 99.30 S19 90.20 15.00 97.50S20 93.40 1.89 99.60 S21 90.30 57.10 43.90 S22 55.30 102.00 4.70 S2348.40 101.00 77.60 S24 61.50 54.80 97.80 S25 75.70 6.60 98.70 S26 76.405.14 99.00 S27 76.80 5.17 99.00 S28 63.20 54.50 98.20 S29 72.20 32.1096.30 S30 75.70 5.35 98.60 S31 69.90 69.40 28.90 S32 47.60 89.70 97.70S33 33.80 102.00 3.90 S34 88.70 0.24 100.00

Example 2

FIG. 5 is a photograph of an example article including an exampleoptical filter and an inked pattern. ESR2 is used as a reflective layer.A ULI layer (sample S01 coating) is applied to the reflective layer asthe wavelength selective scattering layer. A layer of a latex coating(PrintRite DP 261, Lubrizol) is coated on the ULI layer as combinationink receptor layer and sealant layer, which is 1 mil thick when dry. Anink-jet (solvent ink) printed pattern was printed on top of the inkreceptor layer. As shown in FIG. 5, the ink-jet printed pattern is sharpand free of smudges, blurriness, or other defects.

Example 3

FIG. 6A is a photograph of a solar panel. FIG. 6B is a photograph of asolar panel camouflaged by an example optical filter. A multilayeroptical filter was formed by depositing a ULI layer (sample S01) on anESR2 layer. The optical filter was printed with a camouflage pattern(faux wood, similar to the background wood texture). The CIGS (copperindium gallium selenide) film solar panel of FIG. 6A was camouflagedwith the example optical filter, as shown in FIG. 6B. The filter waslaminated to the solar panel with 3M 8211 Optically Clear adhesive. Thecamouflaged film panel generated 45% of its original power. The ESR2film on the back reflected almost all visible light. The power wasmeasured by IV5 solar output test equipment (PV Measurements, Inc.,Boudler Colo.).

Example 4

FIG. 7 is a photograph of an example article including an exampleoptical filter and an inked pattern. The optical filter was formed of anULI layer deposited on a reflective substrate. The right hand side ofthe optical filter was coated with a latex coating (PrintRite DP 261,Lubrizol) that formed a transparent film after drying, as an inkreceptive layer region. A pattern was inkjet-printed onto theink-receptive coated region and the uncoated optical filter region. Asshown in FIG. 7, the quality of printing on the uncoated region on theleft was poorer than in the region coated with the ink-receptive layeron the right. For example, the printed pattern on the uncoated regionwas fuzzy and striated.

Example 5

FIG. 8A-8C are photographs of an example system including an exampleoptical filter and a near-infrared LED (similar to the example opticalsystem shown in FIG. 2E). A structure including a near-infrared emittingLED is shown in FIG. 8A. The structure was covered by an example opticalfilter including a layer of ULI (sample S01) coated on an ESR2 layer.The covered structure was imaged using an infrared camera, resulting inthe infrared image shown in FIG. 8B. As shown in FIG. 8B, the image ofthe LED source is relatively clear, in contrast with the unclearinfrared image shown in FIG. 8C. Unlike FIG. 8B, the structure in FIG.8C (sample S06) was coated with a beaded layer instead of an opticalfilter including a wavelength selective scattering layer. As shown inFIG. 8C, the unselective beaded layer transmitted the image of the IRLED with very poor clarity.

Example 6

FIG. 9 is an atomic force microscopy (AFM) photograph of a surface of anexample optical filter. The optical filter included a surface texturedfilm (sample S05).

Example 7

FIGS. 10A and 10B are scanning electron microscopy (SEM) photographs ofexample optical filters. FIG. 10A shows an optical filter including ahigh haze low clarity ULI layer (sample S22), while FIG. 10B shows anoptical filter including a high haze high clarity ULI layer (sampleS02).

Example 8

FIG. 11 is a chart presenting % reflectance and % transmittance versuswavelength for example optical filters. Curve 72 represents %transmission of a first sample ULI layer (sample S01). Curve 74represents % transmittance of a second sample ULI layer (sample S01, but50% thicker). Curve 76 represents % transmittance of the first sampleULI layer. Curve 78 represents % reflectance of the second sample ULIlayer. As shown in FIG. 11, both sample ULI layers selective reflectedvisible wavelengths, while transmitting near-infrared wavelengths.

Example 9

FIGS. 12A and 12B are charts presenting % transmittance versuswavelength for example optical filters. FIG. 12A presents %transmittance for a first sample optical filter including ESR2 coatedwith beads (sample S06), and limited with PET. FIG. 12B presentstransmittance for a second sample optical filter including ESR2 coatedwith ULI, and laminated with PET. While both sample optical filterstransmitted near-infrared wavelengths, as shown in FIGS. 12A and 12B,the ULI-coated ESR selectively blocked the transmission of visiblewavelengths compared to the bead-coated ESR, which blocked visiblewavelengths to a lower extent.

Example 10

FIG. 13 is a chart presenting % transmittance versus wavelength forsample films. The uppermost curve presents % transmittance for uncoatedPET, which can be seen to be relatively flat across the visible andnear-infrared regions of the spectrum. The middle curve and the lowercurve present % transmittance for a #3 Mayer Bar bead-coated PET layer,and a #10 Mayer Bar bead-coated PET layer respectively. While thebead-coat reduced transmittance, it did not selectively reducetransmittance, and the resulting transmittance curve was also relativelyflat across the visible and near-infrared regions of the spectrum. Thus,bead-coated PET did not perform well as wavelength selective scatteringlayers formed by coating ULI.

Example 11

FIG. 14 is a chart presenting results of Mie scattering, showingscattering efficiency versus wavelength for optical filters includingparticles of different sizes. For optical filters including particlesdispersed in a medium, a model based on Mie scattering was prepared forscattering efficiency as a function of particle size of particlesdispersed in the medium and the difference between refractive indices ofthe medium and the particles. The model was evaluated by setting therefractive index of the medium to 1.5, and that of the scatteringparticles to 1.0. The particle size was varied from 0.2 μm to 1.0 μm, insteps of 0.1 μm (curves from left to right).

Example 12

FIG. 15 is a chart presenting near-infrared scattering ratio as afunction of particle diameter and refractive index difference foroptical filters including a medium and a plurality of particlesdispersed in the medium. The effect of particle size and the differencebetween the refractive indices of the medium and the particle on thenear-infrared scattering ratio was evaluated using a model, and theresults of the model are presented in FIG. 15. The X axis representsdifference between refractive indices (media-particle) and the Y axisrepresents particle diameters (in microns). The contour lines representdifferent scattering ratios such as 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4,1.6, and 1.8. Thus, curve 82 represents a near-infrared scattering ratioof 0.2. Curve 84 represents a near-infrared scattering ratio of 0.4.Curve 86 represents a near-infrared scattering ratio of 0.6. Curve 88represents a near-infrared scattering ratio of 0.8.

Example 13

TABLE 3 presents the minimum scattering (transmission) of the diffusivecoating that can simulate a hybrid surface (or non-metal) with certainrefractive index, on air interface.

TABLE 3 Hybrid or non-metal surface Theoretical max Diffusive coating RIR % SPEX/SPIN SPIN − SPEX T % SPEX/SPIN 1.5 4% 96% 4% 80.0% 1.7 7% 93%7% 73.5% 1.8 8% 92% 8% 71.7% 2.0 10% 90% 10% 68.4% 2.3 16% 84% 16% 60.0%2.4 17% 83% 17% 58.8% 2.5 18% 82% 18% 57.6%

The surface is treated as being white. R % is calculated by Fresnelreflection of air to the material with known RI. The theoretical maximumratio of SPEX/SPIN (diffusive/total visible reflection) was calculatedassuming 100% total reflection=Fresnel reflection+diffusive reflection.

Example 14

The diffusive and total reflectance for a number of samples was measuredusing an X-Rite. The results are presented in TABLE 4.

TABLE 4 Sample R % SPIN R % SPEX SPEX/SPIN SPIN − SPEX I-Phone coverwhite 68.7 67.19 0.98 1.51 White china plate #1 69.29 62.38 0.90 6.91White china plate #2 87.1 83.17 0.95 3.93 White Board 81.29 76.68 0.944.61 Di-Noc white HG-1205 (3M) 89.73 85.65 0.95 4.08 Di-Noc blackHG-1201 (3M) 4.97 0.42 0.08 4.55 Di-Noc gray HG-1512 (3M) 11.42 6.820.60 4.6 Di-Noc red HG-1511 (3M) 12.47 8.08 0.65 4.39

Example 15

Wet-out of near-infrared films was evaluated. A wet-out is a visibledisruption or disturbance in the uniform appearance of an optical filmapplied to a substrate, in particular, at regions where the optical filmcontacts the substrate. Two near-infrared films were prepared byapplying a wavelength selective infrared light transmissive visiblelight blocking scattering ULI layer on a reflective multilayer opticalESR2 film. A near-infrared transmissive black ink was applied to one ofthe films. FIGS. 16A-16D are photographs comparing wet out of thenear-infrared films. FIG. 16A shows the front and FIG. 16B shows theback of the respective near-infrared films, one without thenear-infrared ink coating and one with the near-infrared ink coating. Adouble-sided tape was applied to the back of both films, and the filmswere respectively adhered to respective glass slides. FIG. 16C shows thefront and FIG. 16D shows the back of the respective near-infrared films,one without the near-infrared ink coating and one with the near-infraredink coating, each adhered to glass slides with a double-sided tape. Asseen by comparing FIGS. 16A and 16C, the near-infrared film without thenear-infrared ink coating exhibited visible wet-out, while thenear-infrared film with the near-infrared ink coating appeared uniformand did not exhibit wet-out. FIG. 17 is a chart presenting %transmittance versus wavelength for the near-infrared films of FIGS.16A-16D. As shown in FIG. 17, curve 92 represents the transmissionspectrum for the near-infrared film without the near-infrared black inkcoating, while curve 94 represents the transmission spectrum for thenear-infrared film with the near-infrared black ink coating. Thus,applying the near-infrared black ink coating did not significantlyimpact the visible light blocking and infrared transmittance of thenear-infrared film, since each respective film continued to blocktransmission of wavelengths below about 800 nm, while transmittingwavelengths above about 800 nm. Thus, wet-out was eliminated withoutaffecting the near-infrared filtering properties of the near-infraredfilm.

Example 16

A colored dye was applied to near-infrared films. FIGS. 18A-18B arephotographs of example near-infrared films including a colored dyelayer. Near-infrared films were prepared by applying a wavelengthselective infrared light transmissive visible light blocking scatteringULI layer on a reflective multilayer optical ESR2 film. In the exampleof FIG. 18A, a cyan dye was applied on top of the scattering layer, atthe surface away from the reflective film. The dye coating exhibitedvisible non-uniformity, as seen in FIG. 18A. In the example of FIG. 18B,the cyan dye was applied between the scattering layer and the reflectivefilm. The cyan dye layer imparted a visibly uniform cyan tinge to thenear-infrared film, as seen in FIG. 18B.

Example 17

The effect of applying a near-infrared anti-reflective coating on anear-infrared film was evaluated. The transmittance of a reflectivemultilayer optical film coated with a near-infrared antireflectivecoating was compared to a reflective multilayer optical film without aninfrared antireflective coating. FIG. 19 is a chart presenting %transmittance versus wavelength for the reflective multilayer opticalfilm coated with a near-infrared antireflective coating (curve 98)compared to the reflective multilayer optical film without anear-infrared antireflective coating (curve 96). As seen in curve 96,the reflective multilayer optical film presented high order harmonicsoutside of the main reflective band. The harmonic ripples were strongercloser to the main reflective band. As seen in curve 98, applying thenear-infrared antireflective coating increased the transmission andsmoothed out the harmonic ripples.

Example 18

The effect of a near-infrared dye coating on blocking of visible redcomponent emitted by an infrared source by a reflective multilayeroptical film was evaluated. FIG. 20A is a photograph of an examplesystem including an infrared LED with a visible red light component.FIG. 20B is a photograph of an example system including an infrared LEDwith a visible light component filtered by a reflective multilayeroptical film (ESR2) that did not include a dye coating. As seen in FIG.20B, while the ESR2 film reduced the intensity of the visible componentemitted by the infrared LED to some extent, did not completely block thetransmission of the visible component. FIG. 21 is a chart presenting %transmittance versus wavelength for the reflective multilayer opticalfilm (ESR2) without a dye coating. As seen in FIG. 21, while ESR2transmits wavelengths above about 830 nm (including near-infraredwavelengths) and blocks wavelengths below 830 nm (including visiblewavelengths), ESR2 is unable to block all visible wavelengths. Forexample, the transmission spectrum exhibited peaks between 380 and 450nm, and between 550 and 650 nm. FIG. 22 is a chart presenting %transmittance versus wavelength for a reflective multilayer optical filmwith an infrared dye coating compared to comparative optical filterswithout dye coatings. Curves 102 and 106 represent transmittance ofdifferent optical filters that do not include a dye coating. As seen inFIG. 22, the optical filters of curves 102 and 106, while blockingvisible wavelengths to some extent, they did not completely blockvisible components of the spectrum. In contrast, curve 104, whichcompletely blocks visible wavelengths while substantially transmittingnear-infrared wavelengths, represents an ESR2 film including anear-infrared dye coating, MingBo ink IR 9508-A and MingBo ink IR9508-B(available from Mingbo Anti-Forgery Technology (Shenzhen) Co., Ltd.,Guangdong, China). Wavelengths between 380-800 nm are absorbed by MingBoIR ink but wavelengths between 830-900 nm are transmitted. The MingBo IRink was coated on both sides of ESR2 in the example of curve 104. Thetransmission between 380-800 nm was near 0%, while transmission between830-900 nm was higher than 75%. The film of curve 104 was used toconceal an infrared source in an iris scanning apparatus. Thus, applyinga near-infrared dye coating improved the blocking of visible componentsby ESR2, while allowing the transmittance of near-infrared wavelengths.

Example 19

Epolight™ 7527D Visible Opaque Dye, sold by Epolin, Inc. Newark, N.J.was a dye chosen for its low transmission in the visible up to 875 nmand sharp rise to high transmission above 950 nm. The Epolight 7527D wascombined with coating solution consisting of Vitel 2200 copolyester,sold by Bostik, Inc. Wauwatosa, Wis., dissolved in MEK and Toluene atvarious ratios and coating thicknesses onto a 75 um clear PET substrate.Other binders beyond Vitel 2200B could be employed as dye carriers. Thesamples were prepared on a small scale using Mayer rods and dried in alab solvent oven at 80° C. Coating solutions for loadings (low to highdye concentration) in Vitel are given in Tables 1 through 5 both interms of total solution and total solids. Various optical densities canbe made with these solutions by varying coating thickness.

TABLE 5 1. 3.0% Epolight 7527D Ingredient Amount (g) Wt % of total Wt %of solids Epolight 7527D 0.3  0.3% 3.0% MEK 80 80.0% Vitel 2200BCopolyester 9.7  9.7% 97.0% Toluene 10 10.0% Total 100  100% CoatingSolution % Solids 10.0% Coating Appearance Good

TABLE 6 5.9% Epolight 7527D Ingredient Amount (g) Wt % of total Wt % ofsolids Epolight 7527D 0.60  0.6% 5.9% MEK 78.31 79.8% Vitel 2200BCopolyester 9.50  9.7% 94.1% Toluene 9.79 10.0% Total 98.2  100% CoatingSolution % Solids 10.3% Coating Appearance Good

TABLE 7 11.3% Epolight 7527D Ingredient Amount (g) Wt % of total Wt % ofsolids Epolight 7527D 1.19  1.2% 11.3% MEK 76.95 79.3% Vitel 2200BCopolyester 9.33  9.6% 88.7% Toluene 9.62  9.9% Total 97.1  100% CoatingSolution % 10.8% Solids Coating Appearance Fair, undissolved particlesin coating

TABLE 8 20.6% Epolight 7527D Ingredient Amount (g) Wt % of total Wt % ofsolids Epolight 7527D 2.35  2.5% 20.6% MEK 9.05 78.2% Vitel 2200BCopolyester 74.51  9.5% 79.4% Toluene 9.33  9.8% Total 95.2  100%Coating Solution % Solids 12.0% Coating Appearance Fair, undissolvedparticles in coating

TABLE 9 34.6% Epolight 7527D Ingredient Amount (g) Wt % of total Wt % ofsolids Epolight 7527D 4.70  4.9% 34.6% MEK 73.02 76.3% Vitel 2200BCopolyester 8.87  9.3% 65.4% Toluene 9.13  9.5% Total 95.7  100% CoatingSolution % Solids 14.2% Coating Appearance Marginal, many undissolvedparticles in coating

Five film samples of increasing dye loadings were made with thisprocedure where the term dye “loading” is concentration and/or dyethickness. Transmission spectra at normal incidence were measured on aLambda 900 spectrophotometer made by Perkin-Elmer from 350 to 1400 nmare shown as curves 1, 2, 3, 4, 5, of FIG. 23 corresponding to tables1-5 above.

This series of samples demonstrates two fundamental problems with dyes.The first is that the slope of the transition from extinction to NIRtransmission increases significantly resulting in a reduction of NIRtransmission. The second is a poor quality coating at higherconcentrations resulting from the dye not going into, or coming out ofsolution and causing a grainy appearance and scattering. In addition toresulting in a poor visual quality to the coating, scattering can occurin the NIR which will reduce image quality for NIR cameras.

Example 20

In this example, dyes as described in Example 19 were combined with aninterference film. Epolight 7527D dye coating on PET as measured incurve 3 laminated to a mirror film ESR substrate with 8171 OpticallyClear Adhesive from sold by 3M, St. Paul, Minn. External transmissionspectra of the laminate stacks were measured in a Lambda 900spectrophotometer at normal incidence, 20 and 60 degrees from normalincidence as curves 6, 7, and 8, respectively, of FIG. 24.

FIG. 25 shows the same data set as in FIG. 24 but with a scale of 0-1%transmission. One can see that the transmission is between 0.1 toslightly over 0.2% in broad regions of the visible spectrum.

Example 21

Epolight 7527D dye coating on PET as measured in curves 1-5 laminated toan optimized mirror film substrate with 8171 Optically Clear Adhesivesold by 3M, St. Paul, Minn. was modeled. FIG. 26 shows the transmissionof the samples at normal incidence with dye concentrations that variedfrom 3 to 34.7% as in previous examples. The optimized mirror film wasbased on the ESR2 mirror film sold by 3M, St. Paul, Minn. consisting of265 layers of alternating PEN and PMMA resins. The layer profile wasoptimized to increase the overall bandwidth, shift the LBE from ˜420 nmto ˜400 nm and shift the right band-edge from ˜800 nm to ˜860 nm with asharper right band-edge. This allows for higher transmission in the IRand low visible transmission from 400 nm to 700 nm up to 60-deg incidentangle.

FIG. 27 shows the transmittance of s-polarized (Ts) and p-polarized (Tp)light through the mirror only at normal incidence. FIG. 28 shows a plotof the same data as in FIG. 26 but with a scale of 0 to 0.1%transmittance. The sharpening of the right band-edge and the increase inbandwidth come at the expense of higher transmission in the visiblewhere light leakage can be effectively controlled by the addition of thedye. Higher optical density can be achieved when necessary by increasingthe number of bilayers in the mirror film construction.

Average visible transmission for the curves in FIGS. 26 and 28 alongwith associated dye concentration levels are shown in Table 10. Averagevisible transmission levels of up to 5.8% are seen at the lowerconcentration.

TABLE 10 cc %   3%  5.9% 11.3% 20.6% 34.7% Avg Vt 1.89% 0.97% 0.62%0.17% 0.02% 400-700 nm

Example 22

This example modeled the angle of incidence effects for the previousexample.

Tables 11, 12, and 13 respectively show the results for the previousexample for light at normal incidence, 30 degrees, and 60 degrees. Eachtable shows the average visible transmission for each dye concentrationlevel. Note that the angle of minimum visible transmission is not normalincidence. For this particular set, 60 degrees provides the lowestvisible transmission. It would be possible to design a film havinglowest Vt at an angle other than 60 degrees, such as at normalincidence.

TABLE 11 Normal incidence cc %   3%  5.9% 11.3% 20.6% 34.7% Avg Vt 1.89%0.97% 0.62% 0.17% 0.02% 400-700 nm

TABLE 12 30-deg incidence cc %   3%  5.9% 11.3% 20.6% 34.7% Avg Vt 1.89%0.97% 0.62% 0.17% 0.02% 400-700 nm

TABLE 13 60-deg incidence cc %   3%  5.9% 11.3% 20.6% 34.7% Avg Vt 0.53%0.28% 0.18% 0.05% 0.01% 400-700 nm

Example 23

This example modeled the case where the interference film is areflective polarizer. The layer profile is identical to the mirror casebut the materials are now PEN and CoPEN as in the APF reflectingpolarizer film sold by 3M, St. Paul, Minn. FIG. 29 is a graph showingthe pass and block state of the reflective polarizer at normalincidence. The polarizer is designed to be spectrally selective,blocking visible and transmitting NIR at normal incidence.

The reflective polarizer having transmittance as shown in FIG. 29 wascombined with the same Epolin dye as in previous examples. FIG. 30 showsthe transmission of the samples at normal incidence with dyeconcentrations that varied from 3 to 34.7% as in previous examples.Tables 14, 15, and 16 present spectral response and visible transmissionat 0, 30, and 60 degree angles of incidence, respectively. As before,tables of concentration and average visible transmission are presented.This example shows that even with a relatively high visible transmissioninterference film when combined with the appropriate dye concentrationVt as low as 10%, 5%, or even 1% is possible.

TABLE 14 Normal incidence cc %    3%  5.9%  11.3% 20.6% 34.7% Avg Vt35.20% 17.73% 11.17% 2.92% 0.38% 400-700 nm

TABLE 15 30-deg incidence cc %    3%  5.9%  11.3% 20.6% 34.7% Avg Vt33.08% 16.69% 10.52% 2.75% 0.36% 400-700 nm

TABLE 16 60-deg incidence cc %    3%  5.9% 11.3% 20.6% 34.7% Avg Vt24.66% 12.44% 7.84% 2.05% 0.27% 400-700 nm

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

It is to be understood that even though numerous characteristics ofvarious embodiments have been set forth in the foregoing description,together with details of the structure and function of variousembodiments, this detailed description is illustrative only, and changesmay be made in detail, especially in matters of structure andarrangements of parts illustrated by the various embodiments to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

1. An article that includes an optical filter, the optical filtercomprising: a wavelength selective reflective layer; and at least onewavelength selective absorbing layer, wherein the optical filter hasaverage visible transmittance for wavelengths between 400 nm-700 nm lessthan about 30% and average near infrared transmittance for wavelengthsbetween 830 nm-900 nm greater than about 30%.
 2. The article of claim 1,wherein average visible transmittance for wavelengths between 400 nm-700nm is less than about 5%.
 3. The article of claim 1, wherein averagenear infrared transmittance for wavelengths between 830 nm-900 nm isgreater than about 50%.
 4. The article of claim 1, wherein thewavelength selective reflective layer comprises an interference filter.5. The article of claim 1, wherein the wavelength selective reflectivelayer comprises a multilayer optical film.
 6. The article of claim 1,wherein the wavelength selective reflective layer comprises a reflectivepolarizer.
 7. The article of claim 1, wherein the wavelength selectiveabsorbing layer comprises one or both of a wavelength selective dye anda wavelength selective pigment. 8-9. (canceled)
 10. The article of claim1, wherein the at least one wavelength selective absorbing layercomprises a first wavelength selective absorbing layer and a secondwavelength selective absorbing layer, and the wavelength selectivereflective layer is between the first wavelength selective absorbinglayer and the second wavelength selective absorbing layer. 11-13.(canceled)
 14. The article of claim 1, wherein the optical filtercomprises a sealant layer. 15-16. (canceled)
 17. The article of claim 1,wherein the wavelength selective absorbing layer scatters less thanabout 50% of wavelengths between 400 nm-700 nm and scatters less thanabout 50% of wavelengths between 830 nm and 900 nm.
 18. (canceled) 19.The article of claim 1, wherein the wavelength selective absorbing layerscatters more light in visible wavelengths between 400 nm-700 nmcompared to light scattered in near infrared wavelengths between 830 nmand 900 nm.
 20. The article of claim 1, wherein the wavelength selectiveabsorbing layer scatters more light in visible wavelengths between 400nm-700 nm compared to light scattered in near infrared wavelengthsbetween 800 nm and 1200 nm.
 21. The article of claim 1, wherein thewavelength selective absorbing layer scatters more light in visiblewavelengths between 400 nm-700 nm compared to light scattered in nearinfrared wavelengths between 900 nm and 980 nm.
 22. The article of claim1, wherein the wavelength selective absorbing layer is a printed layer.23. The article of claim 1, wherein the wavelength selective absorbinglayer comprises a coating on another layer.
 24. The article of claim 1,further comprising a wavelength selective scattering layer. 25-27.(canceled)
 28. The article of claim 1, wherein the optical filter isflexible.
 29. The article of claim 1, wherein the optical filter has athree dimensional shape.
 30. The article of claim 1, further comprisinga substrate. 31-32. (canceled)
 33. The article of claim 1, wherein thearticle has a three dimensional shape and includes one or moreattachment features configured to attach the article to an object.34-35. (canceled)
 36. The article of claim 1, wherein the optical filterhas visible transmittance for all wavelengths between 400 nm-700 nm lessthan about 30% and near infrared transmittance for all wavelengthsbetween 830 nm to 900 nm greater than about 30%.
 37. The article ofclaim 1, wherein the optical filter has average near infraredtransmittance for wavelengths between 800 nm-1200 nm greater than about30%.
 38. (canceled)
 39. The article of claim 1, wherein visibletransmittance of the optical filter at normal incidence is less thanvisible transmittance of the optical filter at an oblique angle.
 40. Thearticle of claim 1, wherein visible transmittance of the optical filterat an oblique angle is less than visible transmittance of the opticalfilter at normal incidence.
 41. The article of claim 1, wherein visibletransmittance of the optical filter at an oblique angle between 0 and 60degrees is less than visible transmittance of the optical filter atnormal incidence.
 42. An article that includes an optical filter, theoptical filter comprising: a wavelength selective reflective layer; andat least one wavelength selective absorbing layer, wherein the opticalfilter has average visible transmittance for wavelengths between 400nm-700 nm less than about 30% and average near infrared transmittancefor wavelengths between 900 nm-980 nm greater than about 30%.
 43. Aprinted article that includes an optical filter, the optical filtercomprising: a wavelength selective reflective layer; and at least oneprinted wavelength selective absorbing layer, wherein the optical filterhas average visible transmittance for wavelengths between 400 nm-700 nmof less than about 30% and average near infrared transmittance forwavelengths between 830 nm-900 nm greater than about 30%. 44-45.(canceled)
 46. A printed article that includes an optical filter, theoptical filter comprising: a wavelength selective reflective layer; andat least one printed wavelength selective absorbing layer, wherein theoptical filter has average visible transmittance for wavelengths between400 nm-700 nm of less than about 30% and average near infraredtransmittance for wavelengths between 900 nm-980 nm greater than about30%.
 47. A printed article that includes an optical filter, the opticalfilter comprising: a wavelength selective reflective layer; and at leastone printed wavelength selective absorbing layer, wherein the opticalfilter has a visible transmittance for all wavelengths between 400nm-700 nm of less than about 30% and a near infrared transmittance forall wavelengths at 900 nm-980 nm greater than about 30%.
 48. A systemcomprising: an object; and an optical filter adjacent to the object, theoptical filter comprising: a wavelength selective reflective layer; andat least one wavelength selective absorbing layer, wherein the opticalfilter has average visible transmittance for wavelengths between 400nm-700 nm less than about 30% and average near infrared transmittancefor wavelengths between 830 nm-900 nm greater than about 30%. 49-63.(canceled)
 64. An article that includes an optical filter, the opticalfilter comprising: a wavelength selective reflective layer; and at leastone wavelength selective absorbing layer having average visibleabsorption for wavelengths between 400 nm-700 nm greater than about 30%,wherein the optical filter has average near infrared transmittance forwavelengths between 830 nm-900 nm greater than about 30%. 65-70.(canceled)
 71. An article that includes an optical filter, the opticalfilter comprising: a wavelength selective reflective layer; and at leastone wavelength selective absorbing layer having average visibleabsorption for wavelengths between 400 nm-700 nm greater than about 30%,wherein the optical filter has average near infrared transmittance forwavelengths between 900 nm-980 nm greater than about 30%.
 72. An articlethat includes an optical filter, the optical filter comprising: awavelength selective reflective layer; and at least one wavelengthselective absorbing layer having average visible absorption forwavelengths between 400 nm-700 nm greater than about 30%, wherein theoptical filter has average near infrared transmittance for wavelengthsbetween 800 nm-1200 nm greater than about 30%.
 73. A system comprising:an object; and an optical filter adjacent to the object, the opticalfilter comprising: a wavelength selective reflective layer havingaverage near infrared transmittance for wavelengths between 830 nm-900nm greater than about 30%; and at least one wavelength selectiveabsorbing layer having average visible absorption at 400 nm-700 nmgreater than about 30% and average near infrared transmittance forwavelengths between 830 nm-900 nm greater than about 30%. 74-80.(canceled)
 81. An article that includes an optical filter, the opticalfilter comprising: a wavelength selective scattering layer comprising atleast one of a dye and a pigment, the wavelength selective scatteringlayer configured to scatter visible wavelengths between 400 nm-700 nmand to transmit near-infrared wavelengths between 830 nm-900 nm; and awavelength selective reflective layer configured to transmitnear-infrared wavelengths between 830 nm-900 nm. 82-86. (canceled)
 87. Amethod of making an optical filter comprising: forming a wavelengthselective absorbing layer and a wavelength selective reflective layer,wherein the optical filter has average visible transmittance forwavelengths between 400 nm-700 nm of less than about 30% and averagenear infrared transmittance for wavelengths between 830 nm to 900 nmgreater than about 30%. 88-96. (canceled)